I have done some searching in the forums for similar problems but no one had the same situation, or problems. I have a '70 Chevelle Malibu that had 4 wheel power drums. I bought the complete 4 wheel disc conversion from eBay (same kit I have seen for sale other places) with everything from new master, booster, lines, prop valve, spindles, shields, brackets calipers, rotors and pads. I have everything installed and have bled it multiple times. The pedal pumps up ok without the engine running. As soon as I add vacuum, the pedal mushes almost all the way to the floor, and when I let off, it takes like 4 seconds for it to come back to the top.

-The front calipers seem to have way more pressure when bleeding, the rears kinda leak out

-All the lines are new EXCEPT the long line from the back of the prop valve to the rubber hose which connects to the T on the rear axle.

- All the parts are new so how do I start eliminating components? Its a lot to read so thanks for the time.

Side note: The car is up on jackstands so I started it, put it in first and mashed the pedal. It didn't even load the engine. I also notices fluid dripping from the back of the master down the front of the booster. Is this thing shot?

Side note: The car is up on jackstands so I started it, put it in first and mashed the pedal. It didn't even load the engine. I also notices fluid dripping from the back of the master down the front of the booster. Is this thing shot?

Sounds like it is.

Master is in need of replacing.

Prop valve probably shut off the rear brakes when you mashed it.

Have a similar issue with my 66 Elky----need to re-center that valve.

Ok, I was looking into masters. Wilwood makes them, strange engineering has one or I can go like CPP or ebay chrome. Who makes a good working master that I know will do its job, regardless of looks? I see the popular bore size is 1 1/8". Would a different size be an advantage?

I took this video 20 minutes ago. The car was run several hours ago. I removed the master from the booster and this is what I found. The brown liquid is rusty brake fluid. Strange for a new setup I thought. http://www.youtube.com/watch?v=pzicKCs0yps

With the research I have done so far, I could buy a new 11" single diaphragm booster, or a 7,8 or 9" dual diaphragm booster. Everyone says dual need more vacuum, then someone posts how theirs works fine at 12. I am thinking about the 9" dual (from CPP) because it has the most surface area by far. As far ad the master goes, I guess I need a 1 1/8" bore because I have four wheel disc. I don't know if a stock style unit like I have is shallow or deep pocket, or what single/ dual bail means. If I did I'd probly buy the "right stuff" master from jegs. Can anyone tell me if I'm even barking up the right tree?

Your master cylinder is a dual bail (the wires that hold the cap on). The booster has a long rod so will require a master cylinder with a deep hole in the piston. If the brake fluid did not get into the vacuum chamber the booster will be fine to reuse and a new master cylinder can be purchased at your local auto parts dealer. Buy a new one not a reman, I am using one from a Corvette that looks like yours.

http://i80.photobucket.com/albums/j199/64elcamino/77%20Nova%20project/brakes/linelock.jpg

Ok, I bought a brand new Master cylinder and took plenty of time to do a bench bleed I was confident in. Installed it: Front calipers seemed to get plenty of fluid, but the rears would barely dribble out fluid when bled. I had more fluid running down the front of the booster so I separated the master from it. I just happened to try and pull the rod out, and the plastic sleeve came with it, being snapped right at the back of the rod.

How the heck could that have happened?

I had the original booster from the car upstairs so I put it on. It didnt seem to have the same amount of travel as the new one did, and the pedal was just pure mush. Lastly, it was sucking air like no tomorrow from where the master bolts on.

Is it remotely possible the master still isnt working?

Are boosters supposed to be sealed by the back of the master cylinder?

any opinions would be great, thanks.

Master is in need of replacing.

Prop valve probably shut off the rear brakes when you mashed it.

Have a similar issue with my 66 Elky----need to re-center that valve.


Re-read about the proportioning valve.

Sometimes, you can loosen a front brake line, ---- MASH the pedal real fast and quickly release it (This means a quick HARD jab) and you may hear the valve pop back to center.

Do not stomp on the pedal when bleeding brakes----makes that valve go off-center

Ok thanks Ill try that first, but I ordered an adjustable proportioning valve so if this doesnt do anything, I will eliminate the old valve from the equation.

I disagree with the combination valve blocking fluid if the switch piston is not centered. If this were the way it was designed then a dual chambered master cylinder would not be needed. With a system failure one side of the master cylinder is usually empty by the time it comes into the shop. The light will come on indicating a problem before the all the fluid is gone, it only takes approximately 50 psi difference to move the piston. The moment the bleeder valve opens will create that pressure difference, with very little loss of fluid. When bleeding the brakes the piston in the combination valve will center itself when the pressures are equalized.

Where do you see any way for these valves to block fluid movement?

http://i80.photobucket.com/albums/j199/64elcamino/77%20Nova%20project/brakes/BrakecombovalveBendix.jpg

http://i80.photobucket.com/albums/j199/64elcamino/77%20Nova%20project/brakes/Brakecombovalve.jpg

Well I know it has to stop some of the fluid because in a hard braking situation if all that fluid was sent to the back, with the weight on the front the back wheels would lock up. Whether it has the ability to completely stop the fluid and become stuck that way im not 100% sure of. I bought an adjustable prop valve from jegs and at least I have a little bit of brakes now.

The back wheels will lock up in a hard braking situation. Only 3 things will prevent rear wheel lock up, a height sensing valve like GM used on light trucks in the late 80's, anti lock brake computer and a driver knowing how to control his vehicle.

BTW do not use both the adjustable and the regular proportioning valves at the same time.

I have done some searching in the forums for sim The pedal pumps up ok without the engine running. As soon as I add vacuum, the pedal mushes almost all the way to the floor, and when I let off, it takes like 4 seconds for it to come back to the top.



I reread this and have one more thing for you to check. The pedal side of the booster where the brake rod goes in has to be open to the atmosphere. In order for the booster to work there is vacuum on the engine side to assist in applying pressure, but air has to move in and out of the other side to allow the diaphragm to move. If the vent opening is covered with anything the booster will not function properly.

as far as your master leaking brake fluid on the back side on the front side of the booster mine does the same thing. I noticed thought that depending on which way I put the top of the master cylinder effects this when I put it on the rear clamp is really hard to put on and when on it doesnt leak if I put it on the other way the front clamp is hard to put on and the rear is really easy to put on. dont know if its suppose to be like that or not but it works on mine.


Also I have a very similar issue as yours I just installed a complete power disc brake conversion however when the rpms go up anything past 1200 the brakes get weak and dont clamp very good but at a low rpm they do good. (Cant even do a brake stand brakes wont hold) I also noticed the combonation valve is stuck. Do you know how to get it unstuck? I noticed the rears have good pressure but the front not so much?

Lastly I noticed the brake line at the front caliper where it mounts with the banjo bolt mounts is leaking I have been told that there is right way and a wrong that it goes on as far as it can go upside down and wont seat properly have you heard this or run into this problem when you did your disc brake swap?

I think you're right about the cover deal, adjusting it made that mysterious leak disappear.
My issue is not RPM related, but have you checked for proper operation of the check valve on the front of the booster?

If its a complete disc system, you should have JUST a proportioning valve. A combination valve has a metering valve in front to delay the operation of the the front brakes in a disc/drum system. If you have the brass prop. valve that I had, you should be able to actually unscrew the back off ( the outlet for the rear line) and be able to see if anything is seized in there.

Yes there is a right and wrong way to put those hoses on. They should be offset one way on the caliper end. You want the end that protrudes AWAY from the hose giving the fitting clearance as not to hit the caliper and make it leak. My fittings also leaked and I thought there may be a big issue. I simply got new copper crush washers and I used my propane torch to lightly anneal them. I heated them until they just started to glow, and let them cool off naturally. After that they sealed up fine.

Currently I have brakes that will stop the car, but the pedal is on the floor in order to do so. I am going to get a pressure reading setup so I can see what is going on.