dfseidel

Good news: Car still stops
Bad news: ever since I got this 1974 with power brakes it has
had a really hard brake pedal. I've used a vacuum guage and the booster
holds a vaccum - and it goes down when the pedal is depressed. When
the engine is running the pedal is a *bit* softer, but still much harder than
seems appropriate. I think the previous owner did a front/rear caliper
replacement along with a new booster. What would be the next thing
to check? (oh, brake fluid resevior is full)



[Modified by dfseidel, 3:40 PM 5/23/2004]

lbell101

I guess you could check the proportioning valve.
But that sure sounds like low vacuum, bad booster, or a leaky check valve.

dfseidel

Hmmm haven't checked the prop. valve. -- Vacuum runs 16+ and the check
valve holds, well, at least when I popped it out after a day or so it still
went "woosh". Is there a test for the vavle or the booster itself (if it holds
a vacuum)? Thanks!

lbell101

Yes, for the check valve...
Start the engine to create the vacuum, shut it off and wait 5 minutes. Then try the brakes. There should still be assist, if not the check valve is probably bad.

For the booster...
Pump the brakes with the engine off. This will bleed the vacuum off. Then hold the pedal down and start the engine. The pressure needed to keep the pedal depressed should lessen. The pedal may even go down some. If no change, then the vacuum is bad.

Larry

dfseidel

What about a test for the proportioning valve?

The Check Valve -- there is almost no assist as it is, but what little is there
is still there in five minutes.

The Booster, engine off - brakes pumped. When engine started, just a
a little bit of assist is evident (pedal moves slightly and is a tiny bit
easier to hold).

:confused: :confused:

lbell101

Those sound good. I'm not sure how to test the valve. But I would think the only way the symptoms you describe would come from the valve is if it was blocked either in front or back. I think a frozen caliper would do it too.
I'd take each wheel off and visually check to see if the caliper is actually working. This should verify both the calipers and the valve for these symptoms.
The only other thing I can think of is just low engine vacuum.

Larry

Sharky Guam

Yep, I noticed in your other thread that you had a problem with the timing changing on its own. Timing affects vacuum a great deal. Also, if your check valve is leaking, a drop in vacuum will be apparent as soon as the engine vacuum drops. So fix the timing problem and then test the check valve. If the booster has been replaced, that should eliminate that as a candidate. I might even guess that when the booster was replaced, the check valve might have been installed improperly. Check to see that it was not replaced backwards.

dfseidel

The brake problem started before the timing went south :)

If I stick a vacuum pump/gauge on the booster line I can charge the
booster. It holds for as long as I had let it sit (10+min). If I hit the
brakes the vacuum drops. What are the failure modes of a brake booster?

The check valve is one of those big barb/small barb 90degree things. The
big barg fits nicely into the booster. It seemed to work correctly when a
vacuum was placed on the engine side, but not from the booster side.

dfseidel

I'll check on the calipers when I get the rest of it back up and going :banghead:

Oh well. How much vacuum does the booster need anyway?

R. Bruno

Never ran into it myself, but I have seen in some threads where there was discussion of the rod that goes into the booster being too long or short due to boosters that weren't an exact match. I wish I was more sure of my facts on this, but check the archives or maybe someone else will chime in that knows about this problem. Good luck. :cheers:

Sharky Guam

You need very little vacuum to feel a difference in the pedal effort. Going back to the statements that you posted, I would almost guess that your booster is working properly. If you still have problems, you might jack up the car one wheel at a time and apply the brakes. Try to spin the wheel both with the brakes not applied and with the brakes applied. Note the differences in the effort it takes to spin the wheel. You should not need to have the motor started to do the test. Somehow, I am suspecting that you have some sort of blockage in the brake line,or maybe even a stuck caliper. But if all the brakes work properly with the test, perhaps it might be something else. It is often common to find a detriorating rubber brake line to be the culprit on brakes that are hard or sticking. I have not seen too many bad proportioning valves on the Vettes, so consider that one last. You did not say how well your brakes worked prior to the problems.

dfseidel

They've always been like this -- I've only had the vette a little while, didn't
have an opportunity to test drive first but got such a good deal on it...

Well it seemed like a good deal at the time :crazy:

377vette

How does the engine idle has it got a really lumpy cam? If so it is possable your engine does not make enough vacum to operate the brakes at idle well.
If this is the case you need to install a vacum resivor there about 60 bucks jegs, summit, local speed shop ect..

dfseidel

Well right now it doesn't idle at all :nonod: (see my other thread)

But when it was, it produced about 16"

Sharky Guam

Well if it does not idle, then chances are you have a vacuum leak.....put the gauge on and confirm this before proceeding any further.......If you don't have vacuum, the booster won't work......

Establish proper vacuum of at least 10 on the gauge first and then work from there to confirm that the booster is not leaking.....listening for leaks......

We can then work from there to confirm that the booster rod is properly pushing on the master cylinder....then we can confirm the proper working condition of the hydraulic brake system if all else fails.

My suspicion is that you are not getting proper vacuum and that we are chasing a lot of other leads......

It is really hard to fix a problem without being there to properly assist. So we might have to start with the first step and take things slowly one step at a time since nothing looks really obvious. I am also beginning to think that you have a bad timing chain since you are also having problems with the timing system and its relation to top dead center. All this could also affecet idle and the vacuum in the motor.

All that I guess I could say at this point is to keep the facts coming and we will try to stick with you and find the problem.....you have a lot of talented guys here to help you diagnose the problem.

R. Bruno

http://forums.corvetteforum.com/zerothread?id=799173

Here is the thread I was talking about regarding the adjustment of the brake rod. Bubba may have messed with yours. It sounds like a similar description. :cheers: