Just installed a manual disk brake kit on my 68 Chevelle. The braking performance is not as good as my old drum brake setup. The pedal is real spongy - it feels like the calipers are barely working. This kit has stock 69-72 disk spindles,calipers and rotors and a 1" bore master cylinder. I took off a 1" bore manual drum/drum master cylinder and replaced it with a manual 1" bore Corvette master cylinder that came in the kit. I have installed a residual pressure valve in the rear brake line but that didn't help make the pedal any better. I have bled the brakes 4 times and don't get any air. This is really getting aggrivating!! I have the same brake setup on my 56 Chevy and it works real well. I even swapped the master cylinder off my 56 to my Chevelle and got no improvement. The residual pressure valve helped a little as now I can lock up the rear brakes but the fronts seem to be just along for the ride. Pedal goes almost to the floor before it will stop in a reasonable manner. Fluid comes out of both front bleed screws and when connected to a jar of brake fluid, no air can be seen while bleeding. I have checked my pedal ratio and it is the same as my 56 Chevy - 5:1 The rear wheel cylinders have not been replaced as they are only a few years old. Do you need a larger dia rear wheel cyl w/ disk brakes? Anyone had any success w/ a 1 1/8" bore m/c?? Only other thing I have not checked is the bore of the calipers, but they look just like 69 Chevelle calipers to me. I have not done a pressure bleed yet. Any other ideas on what I can do to improve the performance before I take it to a shop?

Check the adjustment of the rear shoes.

Rear shoes are adjusted OK. Also checked to see if wheel cylinders are leaking but no problems found.

Why would you use a Corvette Master cylinder? Corvettes are 4 wheel disc. Get a master for a manual Disc Chevelle. Did you change the Prop valve when you changed to Discs?

The Corvette Master seems to be the standard for the disc conversions I have seen (Master Power, OPG,etc,etc.) and is what came in the kit. I did change the prop valve to a disc/drum combination valve that also came with the kit. Also added a residual pressure valve on the rear. What is confusing to me is that I have almost the same setup on my 56 which stops fine, but with my Chevelle its barely streetable. I even swapped master cyl w/ my 56 with no change. Only thing I can think of is air in the lines, but I can't find any bubbles. I even bled the system w/ some worn out pads to push out the piston and then retracted the caliper piston to flush rhe fluid back to dislodge any trapped air bubbles. The only option I can think of is to have it pressure bled at a shop.

For pressure bleeding, get yourself another master cylinder cap. Drill a couple of holes in it and install tire valve stems. Use a bicycle pump to apply pressure and it will push through the system.

Car Craft made a pressure bleeder a while back with a piece of wood, a valve stem, and a C-clamp. Anything like this will work.

Of course don't forget to relieve the pressure when checking the fluid level.

If you installed a modern combination valve I don't think you need that "residual" valve also. With a Disc/Drum set up you should be using a Master cyl set up for that also. Not a Disc/Disc one from a Corvette. Piston bore size and stroke are different between the two~ :eek:

Guys, if what Wheels68 has, is what came in a kit and is working fine on another car as he says, it should work on the chevelle as well... I've done a bit of research and almost all stock style single piston and many pbr dual piston kits are using a disc/disc master cyl for disc/drum applications with a 10lb residual valve.

Did you mount the calipers on the proper sides? Bleeders up so there is no air trapped in the piston bore that can't get out...

Calipers are mounted in correct position. Bled brakes 1 more time - no air bubbles found. Calipers appear to have quite a bit of movement. The brackets are not flexing, but the calipers move quite a bit. Checked wheel bearing adjustment and it is OK. Rotor/tire do not move at all when brakes are pressed, but caliper appears to have quite a bit of movement. Ran out of time today, but tomorrow going to pull calipers and see if anything weird is going on w/ the pads/calipers. Thanks for all the posts.

Isn't there a little button on the proportioning valve that you have to push in when bleeding the brakes?

Peter

My first thought is that you either have the wrong combo valve or your's is bad. Are you sure it's hooked up correctly? Do you have the brake warning light hooked up? If so, do you get a light on when pushing the pedal with car ign. on? Also, sometimes during bleeding, the combo valve can get stuck at one end... it should have a little rubber cap over a reset button on one end... see if it needs resetting. What are you using for brake hoses at the wheels?

That's all I can think to check for now...

Good luck,
Bill C.

One more suggestion... if you want a little better pedal feel, try an MC off a 77 Malibu with manual brakes... would be a 6 cyl. car. These MC's have a 15/16" bore to make better pressure.

Bill C.

Checked the metering valve and its working OK. I think I found my problem. The pads on the drivers side are not sitting square to the rotor. The outer pad is tight closest to the hub but I can stick a .020 feeler gauge between the pad and the rotor at the top. The pad is wedged causing the caliper to have to move a lot before the pads are square and tight to the rotor. Inboard side is OK. Took pads off and adjusted on the caliper for better fit and pedal got better. Will try a new set of pads tomorrow and see if it makes an improvement in the fit. If not maybe caliper or bracket is not correct. Calipers in kit were loaded and pads seemed to be installed OK. Didn't look too closely to the pads on install as they were loose since system had not been bled. Live and learn I guess.

Only marginal improvement found. Calipers/pads look good, but still have very spongy brakes. Only two options left, pressure bleed, or trade to larger bore master cylinder. I have bled 2 qts through the system with swapping master cyl and all and can't see how there is any air left. Only other theory is that master cyl doesn't have enough volume for calipers, but I have the same components on my 56 Chevy and it works fine, even swapped its master cyl to Chevelle to check. Starting to get really aggrivated. Checked rear brake adjustment and found it OK. Can't find any leaks in system. Wheel cylinders and calipers are new. Running out of ideas. Maybe there's a reason why Chevy didn't offer manual brakes w/ discs on our Chevelles.

Have you tried swapping the combo valve from the truck to your car? Is the pedal going to the floor and still not stopping? Are you sure the brake pedal is returning to the correct fully out position? Do you have a return spring or just letting gravity do it? Manual brakes should have a return spring on the brake pedal to pull it all the way up, otherwise, it can hold the pushrod in slightly which will not let the MC refill with fluid all the way.

But with rear disks, you should have plenty of fluid to operate both fronts and rears. The problem with w larger bore MC is that pedal fell will get even worse. A smaller bore will increase line pressure and make the pedal feel easier... Have you tried the new MC on your working truck to be sure it's good?

I'd still suspect the combo valve and try changing that. Do you still have the old distribution valve from the old drums brakes? Maybe try swapping that in to see if things change. Do you have access to a brake pressure gauge that will give you the pressures at each wheel?

Bill C.

I do have a return spring on the pedal and rod length is correct. There is a slight amount of free-play in pedal so that master cylinder can re-fill. Checked prop valve by plugging port to front brakes at master cylinder and pressing brake pedal. Pedal is firm. Bled brakes and plugged port to rear brakes. Pedal is soft. Metering valve is working. No leaks found in metering valve and the pin in the metering valve sticks out when brake pedal is pressed indicating the valve is working. Can see movement at calipers so it appears they are getting pressure. Don't have a pressure gauge but that's a good idea. Only thing I can think of is that there is an air bubble somewhere or not enough volume. Guess I'll have to invest in a pressure gauge. Thanks BC.

Why not try the combo valve off the other car? It should at least tell you if it is causing the pressure loss.

If you pluged the port going to the rear brakes on the combo valve and pedal is soft, and you've rulled out the master, seems like it has to be the combo valve.

Also, did you check the flex hose in the back?

Originally posted by Wheels68:
or trade to larger bore master cylinder.Don't do that, you need to go to a smaller bore master cylinder. Smaller bore = more pressure at the caliper pistons/wheel cylinders.

Don't see how the combo valve could be bad if plugging the rear port doesn't make any difference. It is a brand new AC-Delco part.

Smaller bore = more pressure at the caliper pistons/wheel cylinders.


and more pedal travel too!

Originally posted by AdamLym:
</font><blockquote>quote:</font><hr />Originally posted by Wheels68:
or trade to larger bore master cylinder.Don't do that, you need to go to a smaller bore master cylinder. Smaller bore = more pressure at the caliper pistons/wheel cylinders. </font>[/QUOTE]I don't think so Adam. You're applying the same amount of pressure with your foot, but to a smaller area. This translates to less hydraulic pressure.

Heres where I stand right now:

1" bore master cylinder from known vehicle that works. Tested master cylinder and it is OK.

All brake hoses are brand new Wagner both front and rear and no leaks or problems found.

New AC Delco proportioning valve checks out OK.

New calipers and wheel cylinders with no leaks. Calipers are installed with fittings upward.

Rear brakes are adjusted correctly.

New brake lines with no leaks at connections.

New residual pressure valve which tests OK and no leaks at connections.

This only leaves two possibilities 1) air somewhere in the system even though I have sent almost two quarts through it with all the tests and swaps. or 2) not enough volume for front calipers although I do not understand why the same components work fine on my other car. I'm going to pressure bleed this weekend with cjlandrys suggestion of using a modified m/c cap. Hopefully this will sort it out. Thanks for all the replys.

Randy, a smaller bore makes more pressure at the calipers. It's the difference in bore diameter between the master and the caliper that creates the pressure. A larger difference creates more pressure and more pedal travel. A smaller difference creates less pressure and less pedal travel.

BC, I would not go to a 1-1/8" master. I always read that they are a bear to stop the car with. From your description it sounds like you still have air. If it was fluid volume the pedal would travel way down but then become solid when it finally did pressurize everything. When I installed front discs they were a pain to bleed. I initially had them working where I could get the pedal to the floor if I really pushed it. I bled some more and still don't think I got all the air out but they work OK now. I have power with a 1-1/8" bore master so I have a little more volume to work with.

Peter

Randy - I used to think it worked the same way until we started covering it in my physics class. Check it out here:

http://www.infodotinc.com/machines/10d.htm

Finally got the problem fixed. After checking everything over and over, I could not find anything wrong with my system. Decided to give a 1 1/8" master cylinder a try. Did some research and found a m/c for a 69 chevelle w/ power brakes has a deep pocket in the piston which is almost identidal to my manual brake m/c. Bolted the 1 1/8 bore m/c to my system and it brakes excellent. Pedal is a bit firmer than my 56 chevy but it definitely stops the car. Did a remeasure on my pedal ratio and my 68 is closer to 6:1 than 5:1 like my chevy. I should have measured to where my foot hits instead of where brkt attaches to the pedal. This car is an automatic w/ factory manual drum brakes so pedal ratio may vary w/ year. With the 1" bore m/c it was not moving enough volume to the front calipers due to the pedal ratio. Next is to install a booster as daily driving is a little more of a workout, but at least it is driveable. A 1 1/16" bore m/c -if you can find one -would probably be best, but I'm going to go to power brakes when I have the time to bend some new lines.