wcsinx

Title says it all.

I have a solid pedal. It does not sink to the floor with a constant, hard application of the brakes. (i.e. The MC is not internally bypassing)

The booster appears to be good. I can pump up the brakes, hold the pedal down, crank up the engine, and the pedal sinks about 1".

Flex lines are all new.

No leaking calipers, I have never lost any brake fluid in this car.

Bench bled the MC. Bled every wheel in the right (weird) order.

But I still just can't lock up the wheels. I'll get the pedal about halfway to the floor, and it gets hard as hell. But no matter how much harder I push, I get no more braking power, and the wheels do not lock. (and the light comes on sometimes) So it really seems to me that the only thing left is the MC.

Now with the cap off the MC, if I stab the pedal I get a nice little spurt of fluid out of each reservoir. Is this normal? Or does it indicate something nasty?

wcsinx

TTT



someone knows!

MEGALADON

DID you vaccum bleed the brakes?
That never worked for me, and the light still comes on.
Go buy a vac, bleeder pump from your local auto parts
or take it to a shop. then you can lock them up.
worked for me, did it myself to. Good luck

It's 2 in 1 vaccum/brake bleeder. $40.00
You dont need a person pumping the brakes
for you either.

75 Hot One

And so the problem continues. I had the master cylinder rebuilt with SS liner at SSBC but no improvement. You wonlt be able to push the pedal to the floor. On mine anyway, it bottoms about half way down. I checked my booster with a MityVac and it holds vacuum so I don't believe that line about the booster being the problem. As I said in another thread, I've bled about 5 or 6 times with both the MityVac and the pedal procedure in both the nearest-to-farthest sequence and farthest-to-nearest and I still have a soft pedal and can't lock up the brakes, although they do stop fairly well- I'd say just short of lock-up.
I would say that vacuum bleeding is not the answer.
I've got new SSBC Force 10 calipers, rebuilt master cylinder, working booster and I really don't know what to do next. The system is tight- no leaks at all. The brake light doesn't come on- if it does, I'd say you've got an air bubble in one of the circuits.
I'm very interested in a solution to this problem- and it sounds like I'm not the only one.
wcsinx: do you know what engine vacuum you're running. Mine is pretty low at about 6-7". I have a Comp Cams 292H camshaft and I'm starting to wonder if that may be part of the problem, although it shouldn't prevent the wheels from locking on hard brake application since there's a solid link through the booster once the pedal is pushed hard.

wcsinx

No, I didn't vacuum bleed the brakes. The pedal is rock solid though. It doesn't feel mushy/spongy/springy at all. So I'm pretty sure I got all the air out.

wcsinx

I have no idea about my vacuum. All I know is I can pull the line to the booster, and it sucks really damn hard. How many inches "really damn hard" converts too I dunno, but it's comparable to other (newer) cars I've worked on.

The guys that said a new booster solved their problem all described their pedal sinking to the floor. HOW? Again I don't know because like yourself, I assume there's a hard link from the pedal pushrod to the MC pushrod, but perhaps this isn't the case. Regardless, if the pedal holds up, and you can do the pump & crank test on the booster, then it should be fine. Hence it sounds like we're in the same boat.

I'm just going to bite the bullet and buy a new MC, and see how that affects things. I'll just replace part by part until this mystery is solved. *sigh*

75 Hot One

I checked the shop manual- the cross-section drawing of the booster shows a solid link during brake application and the text states so. So that solves that question.
In your case I'd replace the mastr cylinder as well. I was almost ready to replace the booster but found they were discontinued in 2001 so that prompted me to test mine. Good thing, because it looks good.
But that leaves me with nothing left to do. Maybe I'll take it apart again and do a complete bleed and see what that does.

MEGALADON

I would vaccum pump first, before i would buy a MC.

wcsinx

I can get a new MC for the same price as a vacuum bleeder. I'll probably end up doing both.

MEGALADON

I sent you a PM call me

Chris O'

Is it much harder if the booster is diconnected (and blocked)? If the pedal doesn't go to the floor then I'd say everything in front of the booster is fine.

Air in system - pedal to the floor
MC bad - pedal to the floor

To test the MC, clamp off all 4 rubber lines and with your foot hard on the pedal get someone to let a clamp off and reconnect after you see how much the pedal drops. That way you can see which wheel is bad. If you have a spongy pedal with all 4 clamped then it is your MC.

79ls1

There are only three possibilities. Air or blockage in the system, not enough fluid pressure or a bad diverter valve. I'm not very familiar with vette brakes as of yet but usually a brake system is a brake system. By your description, I'm leaning towards trapped air. There is an easy way to find this. With engine off, mash the brakes ONCE and pay attention to where the pedal stops, then mash again, doing the same thing using the same force with your foot. Do this once or twice more, each time notice where the pedal stops. If the brakes "pump up", especially after the second pump, you have air in the lines regardless of how solid the pedal feels. It may pump up a little on the first stroke if you have worn pads, but not after. If it does, all you are doing is compressing the trapped air, which will give a solid feel but ineffective brakes. It can also be sucking air into the system and not leak a noticable amount of fluid. Corroded or worn bleeder screws are the usual culprits (I have had this happen several times). I even had one guy with the same complaint who had accidentally installed the pads backwards (metal backing against the rotors). Don't ask me how. Anyway, good luck, let us know how it turns out.

Forgot to mention to pump the pedal before doing this test to get the vacuum out of the booster.

dath

I've heard of people using this clamp method, but frankly, it scares the crap out of me to go crimping perfectly good rubber lines. . . I'd be afraid that it would cause premature hose failure myself, no?

79ls1

427Hotrod

Not sure why you really want to be able to lock them up. I've made many hard killer stops from WAY over 130mph and it stops fine. Not sure I can really lock mine up either unless I get just crazy. But mine is non power.

Anyway, sounds like you don't have any issue with air in lines. If the pedal feels that good, it is probably OK. As long as there is no restriction in lines, the only thing left is to apply more force to the pistons. This gets into some serious reengineering and must be done correctly.I presume the SSBC calipers work correctly as far as design, but I don't know how many square inches of area they provide or what size pistons are. That really determines braking force and the master cylinder must be able to provide necessary stuff. Past that is there any chance the pedal link is in a wrong hole under the dash and cutting down on your leverage? Not as likely with PB..but something to check.

There are various size cylinders out there. A larger bore one will displace more fluid per stroke, but a smaller one will build more pressure for a given stroke. The pressure in the sytem is constant everywhere at whatever position piston is in. On midyears, the lines themselves were larger on PB cars vs non PB ones.

Sounds like it's possible that master cylinder is not matched to current calipers maybe? Might do some more research on it. Check bore size.

It's normal to have one little burp of bubbles out of reservoirs as the piston passes the hole in bottom of reservoir. But after that, you should not see any more bubbles or fluid movement while holding pedal down.

Have you installed killer race type pads? They are terrible at non race temps and will take three men and a boy to stop when cold. The odds of locking wheels with them cold is almost nil.


You're right on pinching off lines. You NEVER want to do that. It WILL most likely permanently damage the inner liner of hose and it will either allow line to burst or the inner liner will fold over and block line internally.


JIM

Love My 72 and 77

once I diagnosed a bad master cyl. with the guys bumper in front of me

wcsinx

That's a great test, I hadn't thought of that one. Annnnnnnnd ... my brakes passed it. I even just pushed the pedal down with my big toe so I'd be less likely to increase the pressure. And the pedal went right to the same place everytime which is to say just about even with the gas pedal.

Soooo again, I really don't think I have any air in the system. I just ordered a new MC. I'll slap that on there and see if they improve at all along with doing another full bleed. (My old MC looks like crap anyhow) Then the next step is bypass the proportioning valve as a lot of guys have done. We'll see!