[Tim Corcoran]

I purchased a replacement MC for my 65 LeMans bracket car. It's not the factory type it's a light weight aluminum unit and the previously one worked for many years. I have installed three in a row from O'Rileys Auto Parts and all three are bad. The port for the rear brakes won't build pressure but the one for the front brakes does. This is very frustrating how they can peddle this China garbage. I have posted pictures of the one I have so now I am looking for recommendations for a unit from another brand that maybe someone has had good luck with. Thanks.

[Half-Inch Stud]

U-Pull-It from a G-Body, should play, and diagnose until you find new.

[Tim Corcoran]

Not a bad idea I think the one I had on the car for many years was off a junk yard car

[track73]

If your car has discs in the rear try removing the residual pressure valve and see if that fixes the problem. Or if it has drums in backs see if the MC has a RPV.

[Lemans64]

Did you Bench Bleed the Master before installing? Due to the upward tilt of master when bolted to firewall or booster it is almost impossible to get the air out.

[Ben M.]

I was taught to rebuild what I had because of how bad the quality of parts for '60s cars was 15 years ago. Is there any reason why you can't just rebuild the existing one? I would think if the car's been driven regularly it won't have corrosion in the bore and just a simple hone and new seals should do it.

[Half-Inch Stud]

The AL CYL bore ought to look either perfect, or ruined.

[Formulabruce]

Looking at the reservoir, this master is a master for 4 wheel discs. As Track73 said, it could have a reduced pressure valve built into it. This gives it a roughly 70-30 braking ratio, but its make for more flow with less pressure to the rear due to the fact that calipers need more fluid than a wheel cylinder. Converting it could be a hit and miss .
Edit; A 1980, 1981 Firebird NON WS6 master is plastic and runs drum brake rears. The WS7 has drum brakes too,
With plastic / Aluminum master.

[Schurkey]

1. What is that master cylinder intended to fit? Early-80s Dodge pickup? Disc/drum, or disc/disc? Is one reservoir larger than the other, or are they the same capacity? If there's a large reservoir and a small reservoir, are the (Front) disc brakes connected to the port under the larger reservoir, with drum (rear) brakes connected to the port under the smaller reservoir?

2. A residual pressure valve is not going to cause the master to NOT provide pressure. It might cause dragging disc brakes, depending on the residual pressure spring strength, and the sensitivity of the calipers.

3. What is a "reduced" pressure valve, as applied to master cylinders? A proportioning valve, either stand-alone or as part of a combination valve I understand. And I've seen master cylinders that have that sort of valving screwed directly into the cylinder assembly, but this isn't that.

4. Factory defective parts happen. I've seen them, and gotten angry and frustrated as a result...HOWEVER, if I had three in a row that all "failed" the same way, I'd be extremely suspicious of my installation technique. In this case, I'm with Lemans64--HOW WAS THIS BENCH-BLED? HOW WAS THIS FINAL-BLED AFTER INSTALLATION?

[mgarblik]

One thing to check. If it bench bled OK, but did not function correctly on the car, make sure the indentation depth, (counterbore) in the rear piston where it contacts the pushrod is the same depth as the one that used to work correctly. If it is shallow, the push rod won't let the MC piston travel all the way back and the rear section will not refill properly. Then the next brake stroke won't build pressure because there is not a full chamber of fluid.

[Tim Corcoran]

I am bench bleeding the MC. No matter how I bleed the thing the rear port will not pressurize but the front port does. I know how to bleed the brakes properly. There may be air left in the MC but if there is I sure can't get it out. If I hook up bleed lines to both ports and stroke it fluid comes out of the front port just fine but nothing come out of the rear port until the piston is pushed in pretty far but even then not enough to create pressure. If I put plugs in both ports I can still move the piston pretty far into the body.

[Schurkey]

Get yourself a bigass syringe with a rubber tip. Force fluid backwards into the ports, so it starts to fill the reservoir. Do this with the master cylinder tipped DOWN in front at a slight angle--perhaps 5--10 degrees. This way you don't have to rely on the piston developing vacuum in the cylinder to pull brake fluid from the reservoir. Get enough fluid in, you won't need to stroke the piston to bench-bleed, although in this case I'd connect some brake tubing to both ports, with the tube curled-up to dump back into the reservoir and stroke away, just to see if everything works.

https://www.amazon.com/Cardone-10-50...8861141&sr=8-4

[tjs72lemans]

On my 55 Chevy, I've had at least one brand new MC bench bleed as you described. The first one actually had a small pin hole in casting that leaked. My third acted like it wanted to hold the brakes. That ended up being my pushrod to MC needed adjusting back. It was pushing the plunger a bit when installed. Unfortunately, that third one also started leaking at the plunger after about two years. Frustrating trying to get quality parts that work. And last!

[adynes]

Originally Posted by Tim Corcoran

I am bench bleeding the MC. No matter how I bleed the thing the rear port will not pressurize but the front port does. I know how to bleed the brakes properly. There may be air left in the MC but if there is I sure can't get it out. If I hook up bleed lines to both ports and stroke it fluid comes out of the front port just fine but nothing come out of the rear port until the piston is pushed in pretty far but even then not enough to create pressure. If I put plugs in both ports I can still move the piston pretty far into the body.

After the front port is bled successfully, remove it's bleed line and plug it, and proceeded to bench bleed the rear port by itself.

Not sure what method you're using to bleed on the car, but if you are doing the pedal-pump way, instead get a vacuum pump (mityvac, etc). It will change your life. Brake bleeding becomes a simple, quick, one-man job. Be sure to open the bleeder screw fairly far to get a hight fluid flow and pull the air through.

[Tim Corcoran]

First off I want to say that I am not a know it all as I am here asking for help and reading every response for something that may solve this mystery. I don't understand why I would need a power bleeder as I have been doing brake work since the 60's and never needed a power bleeder to successfully bleed the brakes. In fact I normally gravity feed with great success. Recently I have had the wife on the brake while I open and close the valve. I do know the proper method to bleed the brakes. Schurkey, I bought that tool you mentioned in your post and used it to bleed the MC. I bled that MC as many ways as possible. I have literally used at least 6 or 7 quarts of brake fluid while trying to get a hard pedal.

The original MC was installed about 25 years ago and I believe it was a unit I got from a wrecking yard. Recently the car sat for a few years and the brake fluid hardened inside the MC and the piston was frozen and couldn't be moved. That unit looks just like the ones I have been installing that don't work, that unit went into the trash. The plumbing is such that the forward port goes to a line lock then through an adjustable proportioning valve then to the front disc brakes. The rear port the one closest to the firewall goes straight to the rear drum brakes. I drove the car like this for many years and as the car evolved from a low 12 second then to an eleven second street car to a ten second drag car. I raced it for many years like that and the brakes worked just fine stopping the car on the street and from many 130 MPH 1/4 mile runs with no issues. So Bill even if what you say is correct the old MC was plumbed the same way and had a hard pedal and the car stopped just fine. Even if the lines are backwards that would not explain why the pedal goes to the floor with a fully bled MC and full system bled. I have no leaks anywhere, the MC was bled with the plugs in as the method stated. Then bleeder lines going from the outlet ports into the reservoir and cycled many many times with no air bubbles. I am about ready to set the car on fire and push it off a cliff I am so frustrated I want to start racing again and don't know what to try next.

[adynes]

If something caused the master cylinder to gum up, it might have happened elsewhere in the system too. I would pull the lines off at the wheels and blow them out with an air compressor. Getting fluid to move through the system would be very difficult via the pedal pump method, if something is stuck or clogged and there is any air. It will just compress and decompress the air, and not move any fluid.

[johnta1]

Plus the rubber lines (if still have them) could be expanding, making the pedal low.

[455Grandville]

Originally Posted by Ben M.

I was taught to rebuild what I had because of how bad the quality of parts for '60s cars was 15 years ago. Is there any reason why you can't just rebuild the existing one? I would think if the car's been driven regularly it won't have corrosion in the bore and just a simple hone and new seals should do it.

I agree, especially on brake parts made in China.

[Schurkey]

Originally Posted by Tim Corcoran

I normally gravity feed with great success.

Me, too.

Originally Posted by Tim Corcoran

The plumbing is such that the forward port goes to a line lock then through an adjustable proportioning valve then to the front disc brakes. The rear port the one closest to the firewall goes straight to the rear drum brakes.

1. Proportioning valve is not supposed to be used on the FRONT brakes! A proportioning valve is ONLY used on rear brakes, to prevent them from locking up during heavy braking, leading to loss of vehicle directional control. Front disc brakes might get a metering (holdoff) valve, though.

I'm not an expert...but I thought line-locks also went to the rear brakes, to hold the rear wheels when the vehicle was staging. Am I wrong?

2. Where's the safety switch that turns on the dash light when there's a hydraulic failure in one side of the dual-circuit brake hydraulics?

3. You've never answered if the master cylinder has a large and a small reservoir, or two equal-capacity reservoirs. The large reservoir always goes to the front disc brakes; the smaller reservoir goes to rear drum brakes. Two small reservoirs = master intended for 4-wheel drum brakes. Two large reservoirs = master intended for 4-wheel disc brakes.

On GM Quick-take-up masters, the rear reservoir/piston is for the front brakes, and has the larger reservoir.

Originally Posted by Tim Corcoran

I am about ready to set the car on fire and push it off a cliff I am so frustrated I want to start racing again and don't know what to try next.

MAYBE you really have gotten three defective master cylinders in a row. It's possible.

I don't think it's likely, though.

[ELKHORNAOG7]

I have a kit you can find on ebay with fittings and a pressure gauge 3000 psi I would get one of them and hook it up to each wheel cyl. and have someone hit the brakes and see if you are getting pressure to the rear brakes this will tell you if the MC is working or not----BOB