I don't know why but with a new brake M/C specifically for a 1984 Corvette, from Corvette Central, (there were several part numbers of each year with them so I made sure to get the 1984 M/C), I have very little rear brakes. I have this problem problem where the fronts will lock but not the rears. The 1984 does not have antilock brakes. I have a new brake booster and can feel it working. I bench bled the M/C so I know there is no air there. Of course when installing the M/C I do lose just a bit of fluid and there may be a bit of air in the line while the lines are installed. I did bled them thoroughly afterwards. I have done this job countless time on cars I have owned and with dirt track racing so I am familiar with doing this job.

When I have the car on the lift, engine off, the pedal stays the same level after pumping it, so good there. Now I start it up, I feel the booster working. I put the car in D and watch the rear wheel flange turning. I put my foot on the brakes and it slows but really never stops. I pushed really hard on the brakes, I mean like angry hard but they slowed to a crawl but never really stopped!!! This is actually the 3rd M/C I have put on this car with the same results. It is a simple layout for the brakes. 2 lines to the front brakes and 1 to the rears. The rear has a metal T then splits to flexible hoses (new) to the calipers. Very simple. I know there is some sort of proportioning valve in the M/C to give most strength to the fronts but maybe 60/40 or 70/30 but I have never had a set of rear brakes that won't lock up like mine.

Even if the rotor and pads are glazed up they should lock up on the lift. I tried holding pressure on the brakes to heat the rears but made no difference. The fronts lock up easily. When I crack the rear bleeders I get a really good flow from both sides. I was thinking if the calipers were really stiff or stuck then when released they would still hold some pressure on the caliper but when I let off the pedal, the rears run free. The pads are basically new. When I push the pedal the rears act like they will lock up but the pedal has no more travel, seems to release the rears a bit after that initial try with the pedal. Its almost like the fluid to the rear in the M/C is bleeding through the proportioning valve or the inner cups in the M/C itself. But like I said, all the M/C's I have put on the car have acted the same!

The booster isn't leaking air, the 1 way valve works. Pulling the hose off speeds up the engine like it should. I checked for any air leaks, found none. Most boosters work the same.

Any ideas?

Thanks

Russ

 

The proportioning valve? I do know that on older cars it had to be reset .

 

I remember doing that on some older British cars but this being a new M/C, I wouldn't think there would be a problem there. Now if the front or rear of the M/C goes bad then the proportioning valve sensor will move and show a brake fault light on the info center. I now they mostly had to be reset when the M/C was rebuilt too. I don't see anything about resetting this in my owners manual either. It is frustrating for sure. It should be a simple bolt on thing. Especially with no anti lock system to interfere with the installation.

 

The rear brake line travels along the pinch weld to the back. Make sure a jack or jackstand hasn't crimped it.

 

Thanks. I have a lift and have followed the line from the M/C to the rear. It is in very good shape. From the T where the line splits, both lines are in good shape. Then the flex line, both new. The bleeder flows lots of fluid through both calipers. It feels like the rears are initially getting fluid but then it seems to bypass that fluid somehow in the M/C. I have an older car and like most of them, there is no proportioning valve. The M/C has different bores front to rear to do the biasing. Many race cars have either 2 M/C and a balance adjuster to get the feel you need or the percentage from front to rear or a single M/C with an adjuster for each line. The really old days there was only one line out from the M/C to all 4 brakes, usually only drums all around.

It is impossible to test on the bench as to how much pressure you can build up with the 3 lines. Somehow, the rear initializes when you first step on the brake pedal or bench bleed it. But in my brake M/C the rear pressure releases mostly, not stopping the rear rotors but continues until the fronts lock up, keeping ther rears rolling but with some friction on the pads from the M/C. I have never run across this. I would almost like to try an older brake M/C without the proportioning valve, just the one bore from front to back.