Douglas Sorrells

I am having a problem with the brakes on a 86 951. I have recently bled the entire system and know it is properly bled. My pads are fine and have stainless steel all around but still have to stand on the brake to get the car to stop. A friend of mine has tested the vacume at the manifold and said it is good, but there is NO blue and black check valve on the line going to the master cylinder. Is that valve necessary?

reno808

yes i beileve so.. well was it there before?

luckett

Are you sure you bled both sides of the calipers?

Ian Carr

YES! With out that the brake boost does not work and you have manual brakes... I know cause my broke

ibkevin

I'm hoping when you said the brakes have been bled you flushed the system and used quality fluid. Also, replacing the venturi o-ring and tightening up that mess of hoses under the intake made my brakes pretty snappy

Jfrahm

Does the booster hold vacuum? You could, um, suck on the hose to check.

Ian Carr

You guys he needs that little check valve (black and blue thing) or the boost will not work im telling you.

Peckster

Your brakes should be amazing, so you have a real problem.

No idea what it could be though. How long have you had the car, were the brakes ever good? I wonder if the pads are glazed or something.

hosrom_951

That blue/black check valve is for the climate control system flappers (recirculation/fresh air), on stock cars, it sits on the brake booster hose with a T connection. If you have a broken T connection you have a vacuum leak (it may not show, mine didn't).

I replaced the brake booster hose with a direct connection from the brake booster to the intake manifold, eliminating the blue-black check valve (it is connected to another vacuum source).

There is another check valve and a seal on the brake booster itself, did you check that??

cjb3

Hussam--I have been working with Doug on these brakes---last night we bled the brakes again--very thorough and both sides of the calipers front and rear--the system was totally flushed.

I have tested the check valve and even switch it with another car that works--it works fine.

I have also checked the vacuum in the round black diaphram--it holds the vac. well after the car is turned off.

Flat out we are stumped--I have had 7 951's and never had this problem.

We have concluded that we will swap the master and assist diaphram over the weekend---unless someboyd out there has a better idea---Please help--my hands are raw from removing the wheels--and now can remove the entire intake in less than 47 seconds--LOL!

Thanks,

Cyrus

ps---I dropped Doug off at the mental hospital for a 48 hour observation.

hosrom_951

I am sure you did this correctly, but was this how you bleed the system:

Right rear
Left rear
Right front
Left front


Do you notice any small black particles in the brake reservour??

cjb3

Yes--we did start far to near---no black particles---but when we were getting some really small bubble in the brake lines.

What do the black particle mean? I will take a closer look for them.

I found a mention of the same problem on this forum--it appears as though the Master Cyclinder may be shot--this will be alot easier than changing the Vacuum Diaphragm.

Thanks for the help,

Cyrus

Jolly

Actually, first check to see that full vacuum is actually making it to the booster. 2= vacuum booster or master cylinder.

When you apply brakes, are the pedals hard as rocks, or do they mush? If they are hard to push (just like trying to stop a car when the engine's stalled), you are not getting any boost assist= New vacuum booster.

When bleeding can you pump the pedal until it gets hard, or does it just mush. When you open a bleeder screw and pump the pedal, the fluid should shoot with considerable force.

Douglas Sorrells

Yea the brakes are very hard to push with very poor stopping power & the fronts are Big Reds. We bled all from far to rear, inside and out. There is vaccume to the booster and the venturi tube is not leaking. We used dot 4 brake fluid and there are no vaccume leaks under the intake or near the brake booster. My turbo guage shows -20 lbs of vaccume off turbo. I spoke with Tim at Speedforce Racing, and he said to me what Cy has said all along. "The little black & blue value has nothing to do with the brakes and is related to the heating". I tend to agree with Jolly that it's probabally the vaccume booster. We will be swapping the master cylinder out tomorrowand if that's not it, the booster will follow.

Will post results.

Thanks to all for their input!!

Doug

Jolly

I would save time and just go for the booster. There are companies around that will rebuild it.

Hard pedal means no assist. Nothing to do with your fluid/pads/calipers