No rear brakes on my 68 [Archive]

67SS138

Jun 13th, 08, 12:51 AM

This is rather lengthty but informative curtesy of cpp brakes.

Troubleshooting Bleeding Difficulties
If you are having difficulty getting all the air out of the system or you are not obtaining a firm pedal, try these steps to assist in the bleeding process and track down problem areas on the vehicle. A spongy pedal is usually caused by air in the system.

While bleeding, raise the end of the vehicle that is being bled, this may help air trapped in the system to rise towards the bleeder screw.
While bleeding, lightly tap the caliper with a hammer. This can dislodge air bubbles that cling to the walls of the caliper reservoir.
Check for damaged brake hoses that may be internally expanding from the line pressure. (See Check Flexible Brake Hoses)
Use line locks (vice-grips) to restrict fluid flow in the rubber lines at each wheel between the frame and the axle to help pinpoint the trouble spot. Lock all four wheels at the same time. Be aware that crushing the brake lines can damage them. To help prevent this, wrap the jaws of the line locks with a shop towel and close the line locks just enough to restrict the flow of fluid. DO NOT OVER TIGHTEN
With the lines to all four wheels blocked off, check the pedal. You are looking for a firm hard pedal that does not drop under constant pressure over a short period of time. If you get a hard pedal, then the problem is definitely at one of the wheels. If the pedal is still spongy, first make sure the lines are effectively restricted and try the pedal again. If you still do not have a firm pedal, then remove the master cylinder to follow the bench bleed and test procedure.
If you do have a hard pedal, then remove the line locks from the farthest wheel from the master cylinder and press the pedal. If the pedal is spongy again, the wheel you just released is the problem wheel and requires further bleeding. (In rear disc brakes, the problem may also be that the parking brake needs adjustment. Check your parking brake adjustment before proceeding.) Check to make sure that all the steps for bleeding are followed. If you are still having difficulty, we suggest the use of Speed Bleeders to help ensure air does not return in the lines when the pedal is released. Otherwise a power bleeding system may need to be employed or a professional brake specialist.
If the pedal is still firm then continue on to the next wheel in the proper bleeding order, which is the wheel with the next longest line in the system, release the line lock at that wheel and test the pedal again. Note that it is normal to have a small amount of increase in travel as you release the line locks from each brake hose.

Back to Top

•Combination Valves
These valves offer multiple functionality in a single unit. They combine the functions of both a metering valve and a proportioning valve with the addition of a brake light warning switch. The brake light warning switch signals when there is an imbalance of pressure in the system caused by a problem in the front or rear brakes. By far the best valve to use for all disc brake conversions. All our kits use DOT approved Combination Valves designed specifically for either Disc/Disc or Disc/Drum applications.

Back to Top

Check Booster Pushrod Adjustment
The pushrod that actuates the master cylinder must be properly adjusted. Ideally there should be only slight clearance between the booster pushrod and the master cylinder pushrod. Interference will preload the master cylinder. When the system is preloaded, it builds pressure each time the pedal is pressed. Since the master cylinder is not allowed to fully release the pressure from the previous stroke, the system will eventually lock the wheels. Too much clearance will cause excessive freeplay in the pedal. In almost every case, if you bought your booster and master cylinder from us, we pre-adjust the booster pushrod and master cylinder for you so this is not a problem.

To make 100% certain of a properly adjusted booster pushrod for your master, we offer a master cylinder depth gauge (PN: CP2003) to determine correct pushrod length. The following illustration demonstrates the proper usage of this tool. First measure the length that the booster pushrod extends past the face of the booster with the "head" of the pin. on the tool. Then measure the depth of the master cylinder piston with the other end of the pin on the tool. If the pin hits or there is too much space on step two, then adjustment is necessary.

There are basically two different master cylinders that we use for our Chevy and Ford kits, the Deep Bore master cylinder and the Shallow Bore master cylinder. To determine which master cylinder your booster is setup to mate with, you can do the following simple check. For Shallow Bore master cylinders, the pushrod should be approximately flush with or below the booster face. Deep Bore master cylinders will protrude past the face of the booster body by about 1" to 1-1/2". Note that Deep Bore master cylinders are required for manual brakes and Shallow Bore master cylinders are generally used for power brake systems. We now supply a master cylinder piston adapter to accommodate both master cylinders. You will only use this adapter when you have a Deep Bore master cylinder with a short booster pushrod.

Back to Top

Rear Disc Brakes and Parking Brake Adjustment
This is another critical item that many people miss when upgrading to rear disc brakes. Our rear disc brake calipers that are equipped with an parking brake are self-adjusting. Every time you use the parking brake they adjusts themselves for pad wear by clicking to the next stop on the internal ratchet. If you do not use your parking brake during normal operation of the vehicle, over time the pads will wear and there will be insufficient contact between the pads and the parking brake mechanisms. When this happens the parking brake will never engage.

To adjust the parking brake while installing or servicing the calipers, use the following directions. Failure to adjust the parking brake can result in no parking brake, brakes dragging, overheating, premature brake wear or ineffective rear brakes causing excessive front wear and overheating.

If you are adjusting the parking brake after the system has been bled, remove the master cylinder lid and make sure that the fluid level is no more than 1/2 full, this is so that in the following steps when the caliper piston is pressed back, fluid does not overflow the master.
With the caliper in place on the wheel, remove the parking brake spring and lever arm. Remove the seal and nylon washer from the adjusting screw and place them in a clean location.
Turn the adjusting screw counterclockwise to tighten it and collapse the pads until the pads are tight against the rotor.
Note that the adjusting screw clamps the pads closed when tightened counterclockwise. When the adjusting screw is turned counterclockwise past a certain point, it turns the internal ratchet. This is how it is adjusted. Adjusting the screw can be tricky because when the it is tightened all the way, its hex head recedes into the caliper body and you can't get a wrench around it.
Turn the adjusting screw in counterclockwise by hand until there is resistance.
To push the adjusting screw back out to provide access to it's hex, use two channel-locks to squeeze the rear brake pad and compress the caliper piston. Place the wrenches on either side of the pad locating the jaws on the pad bracketry and the body of the caliper.
Then use a wrench to turn the adjusting screw counterclockwise to change the position of its hex and slip the internal ratchet.
Back the adjusting screw out by turning it clockwise, place the lever arm over the adjusting screw hex and apply a medium amount of hand force clockwise to the lever arm to push it past the lever stop on the caliper. Once the force has been applied, the lever should be located within 1/4" of the lever stop and should be easy to put on the adjusting screw head. There should also be mild contact between the pads and the rotor when properly adjusted. It is highly likely that this will need to be done a number of times before it is properly adjusted.
Remove the lever arm and replace the nylon bushing and seal, then replace the lever arm and secure with the nut.
Replace the return spring and parking brake cable

.

Back to Top

Check Pedal Assembly
Something that many people are not aware of when upgrading their system is the brake pedal adjustment. Improper pedal adjustment can do the following:

Preload the booster causing brakes to drag and eventually lock up.
Damage the booster's internal components by actuating at an improper angle.
Allow slop in the pedal if adjusted too low.
Many vehicles have a second hole on the pedal assembly that was originally intended for power brake applications. The required hole is generally 1" to 1-1/2" lower than the original manual brake hole. If there is no second hole, you may need to drill the hole in the pedal arm to properly align the push rod. The easiest way to determine the perfect location for this hole is to find the center of the push rod's vertical travel to ensure that its actuation is as inline with the booster as possible.

Gently lift the end of the pushrod until it stops and noting the location where the clevis on the pushrod locates on the pedal arm. You can mark this location.
Then gently push the pushrod down until it stops. Mark this location as well.
The proper hole location is in the middle between the two marks on the pedal arm. Mark and drill the new hole in the center of the pedal arm. Attach the pushrod to the pedal arm.

Check freeplay in the pedal by applying pressure to the pedal with your hand and noting how far the pedal travels before resistance is felt. It should be approximately 1/4". This freeplay allows the master cylinder piston to return to the "at-rest" position and prevents preloading the system. Too much freeplay will drop the pedal too far before applying brake pressure and may even hit the floor before applying full braking force to the system. Note freeplay is also dependent upon proper adjustment of the booster pushrod. See "Check the Booster Pushrod" below.)

Back to Top

Check Proper Caliper Alignment
The proper alignment of the calipers is critical to a safe brake system and preventing premature wear on the rotors and pads. It may be necessary to grind small protrusions on the caliper to ensure a proper fit and alignment of the calipers on the caliper brackets. DO NOT grind on the caliper brackets, this will weaken the design of the brackets and may lead to brake failure. Be aware that grinding can produce a tremendous amount of heat. Because of the relatively thin amount of material of the plate, subjecting the brackets to this heat may causing them to become brittle and break under stress!

If wheel flange is warped causing the caliper and bracket to not align properly with the rotor as in the figure below on the left, insert one or two washers between the wheel flange and the caliper bracket. Use the washers to shim the assembly and align with the rotor as necessary.

Back to Top

Test Master Cylinder
Note that this test of the master cylinder requires a complete re-bleeding of the system since the brake lines are removed from the master to perform this test. This is why we include this test during the bench bleeding process. However, just because you have performed the test during bench bleeding does not mean that that air has not entered the master cylinder since then. Should air enter the master cylinder at any time after bench bleeding, the master cylinder MUST be removed from the vehicle and bench bled again. A number of things can cause air to enter the master cylinder:

If the fluid levels drop too low during the bleeding process.
If the master cylinder was left sitting for too long after the bench bleeding before having the lines installed.
Mishandling of the master cylinder such as if it were dropped or jarred excessively.

Remove the brake lines from the master cylinder ports.
Block off the master cylinder brake line ports using the correct size inverted flare plugs or bolts with the appropriate thread size for the ports on your master cylinder. Dual port master cylinders that have ports on both sides need to have all four ports plugged. The protruding cone of the inverted flare seat in the master cylinder port is made of a soft material that can easily be deformed if over tightened. If using bolts, be sure to just snug the bolts so as not to damage the cone seal surface. This cone mates with the inverted flare (expanded mouth opening) of the brake lines. If you have the ability, you can also drill a point into the end of the bolt to help prevent this from occurring. Most GM master cylinders use 9/16-18 threads for the front ports and 1/2-20 threads for the rear ports. Most Ford master cylinders use 3/8-24 threads for the front ports and 7/16-24 threads for the rear ports.
Apply constant pressure to the pedal, the pedal should be firm, hard and should not drop over time.
If the pedal is squishy there may be air in the master cylinder. Bench bleed the master according to the instructions above and test again.
If the pedal is firm and then drops over time under constant pressure, the master cylinder should be replaced.

Back to Top

Test Combination/Proportioning Valve
Use a test light by attaching a clip to a positive contact on the vehicle and touch the point of the tester to the electrical connection of the combination valve. If the the light does not come on, the valve system is operating correctly and no further testing is required.
If the light does come on, this indicates that the pressure differential valve is stuck in the front or rear position.
Bleed the brake system to determine if the front or rear lines are blocked off. Set up one front wheel and one rear wheel for bleeding at the same time. Crack both bleeder screws and gently pump the pedal a few times.
The blocked side will trickle fluid out when the bleeder screw is cracked and the pedal pressed. An unblocked line will squirt fluid out the bleeder.
The lines that are clear must be left open and the blocked lines should have the bleeder screws tight to cause pressure to build up on that side. Be sure to use the standard bleeding procedures to prevent air from entering the system.
Slowly press the pedal with steady pressure a number of times until the light goes out; this will center the differential valve. You may also hear a pop come from the proportioning valve. This is the metering valve returning to its equalized position. When the light goes out, close the bleeder screw. (See fig. below)

Back to Top

Test Power Brake Booster
If the pedal feels "hard" while the engine is running, the booster isn't operating correctly. If you suspect the booster is defective, do not attempt to disassemble or repair the power booster. Doing so is unsafe and will void your warranty.

Test 1

With the engine off, pump the brake pedal to remove any residual vacuum in the booster.
Hold pressure on the pedal while you start the engine. When the engine starts, the pedal should drop about a 1/4", this indicates that the booster is working properly.
Test 2

Run the engine a couple of minutes.
Turn the engine off and press the pedal several times slowly. The first pump should be fairly low. The second and third should become slightly firmer. This indicates an airtight booster.
Test 3

Start the engine and press the brake pedal, then stop the engine with the pedal still pressed. If the pedal does not drop after holding the pressure on the pedal for 30 seconds, the booster is airtight.
Inspect the Check Valve

Disconnect the vacuum hose where it connects to the intake manifold. Do not disconnect the vacuum line from the booster. Air should not flow when pressure is applied, but should flow when suction is applied. If air flows in both directions or there is no air flow, the valve needs to be replaced.
Verify Enough Vacuum

Check the operating vacuum pressure when the engine is at normal operating temperature. There should be a minimum of 18 in. of vacuum. Vacuum may be increased by properly tuning the engine, checking for vacuum leaks and blockages in vacuum lines.

Back to Top

Brake Fade
Brake fade is a gradual loss of braking power that can be caused by old brake fluid or overheated brakes. Overheated brakes is a very dangerous situation which can cause old or contaminated brake fluid to boil. The gasses released from boiling brake fluid will cause a squishy pedal even after the brakes have cooled and can cause brake failure if not immediately taken care of. This is all the more reason to ONLY use brake fluid that is from a newly opened sealed container. Drum brakes can compound the problem of overheated brakes. Since the drum itself expands when heated, it increases the amount of travel required by the shoes to effectively stop the vehicle. Brake fade can also occur in drum brakes by the accumulation of water inside the drum since it traps water inside. The water then acts as a lubricant between the shoes and the drum, causing ineffective braking.

Back to Top

Why change to Disc Brakes?
Disc brakes offer a significant advantage over drum brakes in a number of areas, the most important is in safety.
Disc brakes resist heat induced brake fade. The design of disc brakes dissipates heat much more quickly than drum brakes. Also, heat causes the disc to expand which has no effect on braking ability where as the drum expands it increases the amount of travel required for the shoes to apply effective stopping force.
Disc brakes resist water induced brake fade. When disc brakes become wet, the large majority of the water is spun off of the disc during rotation. The residual water evaporates from the heat caused during braking. In a drum brake set up, water can become trapped inside the drum and act as a lubricant between the drum and shoes causing water induced brake fade.
Disc brakes are better at straight-line stops. Drum brakes have a tendency to pull due to inconsistent alignment of the shoes from the left to right wheels due to a dependency on multiple complex floating mechanisms. These inconsistencies can cause the car to veer unexpectedly to one side or the other during panic braking. Since disc brakes apply equal force through clamping, they are much safer during straight line braking.
Ease of serviceability. Disc brakes are much easier to service than drum brakes.
Lighter weight
Recommend the addition of a power brake booster for disc brakes.

Back to Top

© Classic Performance Products Inc.