I have been reading the others for a couple of evenings and can not seem to find the answer I am looking for maybe it is too obvious or basic to everyone else..LOL
Car 91 L98
Problem brakes seem as though they do not stop as they should
Can not lock brakes or set off ABS on a dry road

Things done so far
New master cylinder (actually rebuilt from NAPA) and did a proper bench bleed and installed a drm bias spring at the same time.
New rotors GM from a forum sponsor.
rebuilt calipers
stainless brake hoses
hawk hps pads
everything carefully cleaned during installation no oil hand prints etc.
Flushed brake fluid without letting air get in and went through about 2 quarts of fluid.
Vacuum test booster and seemed like not leaking but not really sure that proves anything.

The abs and brake light do cycle like they should and abs light does not come on while driving other than the one time at 4 mph. I can not hear the abs cycle on startup but I have a borla exhaust and about 50% deaf from 35 years at race tracks with open headers and the same time in dyno cells and do have trouble understanding people that are talking and not facing me so suspect I read lips to some degree.

There are only 2 parts left to my knowledge..
1 Brake booster
2 ABS system

I am leaning towards the booster for the following reason. I have a 2002 Pontiac gran prix and while not a Corvette it does have 4 wheel disc brakes and a power booster. In the Pontiac I get about 2 1/2 inches of soft travel progressivly getting stiffer and in the Corvette there is about an inch between soft and rock solid. Pontiac I get 2 power assisted pumps after I turn the engine off Corvette 1 at best. In all honesty I would rather play with relays and fuses than crawl way under the dash to change the booster but I don't want to sell myself down the road of playing endlessly either. I have followed the threads about brakes and lots of pro's and con's on which booster to use if this is the problem. I am not going to buy a GM one no way are that many $$$ to justify a GM only this is a driver a very clean one but still a driver. Cardone is easy and assume they are rebuilt plastic ones. I have no plans for headers in the near future and the original lasted around 20 years in plastic. Thoughts? A I missing something here?

OK Jake this is a real post this is not a test..LOL I hope I made it very clear and hope everyone else does too...
Dave

 

No help on what the brake pedal 'feel' should be like? Does it act like any other car with power brakes where it is initially pretty soft and gets more firm for a couple of inches then finally get a hard pedal? This is the only Corvette I have every owned so I have no idea of what 'right' is. I am thinking that I can not lock the brakes or set off abs on a dry road is I don't have enough braking power to do it.
Dave

 

hard brake pedal usually means bad brake booster. when you push the brake pedal, can you head a whooshing noise from that area? sometimes you can, sometimes you cant with a bad booster. on my car the booster was leaking bad and i had a hard pedal that didnt stop well. you can look on the c4 general discussion, in the stickies there is a tech tips. i did a write up on replacing the booster with pics and all

 

The "power" brakes on my 92 are different from my wife's 04 and our 2010 Tahoe.
93 has J55 brakes, new booster, master, calipers last winter.
The difference between no brake and I'm slowing down is not very much pedal movement, A little more movement and ABS is on with street tires, with the R compund tires I can almost get to the floor before the ABS comes on. But by that time I'm almost at a stop from 100+ Yep, I'm talking about on the race track, and carbotech xp12/10 combo.

Wife's C5 has a good replacement pad and has a lot more pedal movement from no brake to full brake, its the pads in this case.

The truck I'm at 50% pedal before I feel the brakes are starting to work.

 

I found the bleed order to matter on our 91 ZR-1: RR, LR, LF, RF. Ran through several pints in a couple of frustrating bleed-fests before RTFM'ing. Doh.

Best,
- Jeff

 

If the rotors and pads are brand new, maybe they need to be bedded in?

 

Pedal is hard so seems like they are bled ok nothing spongy or soft and bedded them in according to the sheet that came with the pads. I did a vacuum test on the booster and it holds just fine. My problem as I see it is that there is not much difference between engine running or not and there is only 1 inch between pedal movement and a hard pedal and poor braking with the hard pedal. My 02 Pontiac will outstop this thing easily or at least with more confidance and way less pedal pressure to do it. I don't mind buying a booster though not looking forward to the install of it but equally not looking forward to playing around with the ABS mysteries either.

 

My 86 had the same symptoms, could not lock brakes or even get the abs to cycle on dry roads. ABS did cycle on start up and light was normal. After several years of changing out parts I did the c5 zo6 brakes and still lousy brakes. Finally out of desperation I pulled the abs fuse and at 40mph, both feet on the brake pedal and stood up with all my weight on the pedal. Brakes locked and after the tire smoke cleared I reinstalled the abs fuse. Now I have great brakes like a Corvette should have. My guess is something was sticking in the abs and it took a lot of pressure to shake it lose.

 

How hard are you trying? My first attempt to actuate the ABS didn't work either, since I didn't get to lock-up even with the same pedal pressure that would have seen me ejected through the windshield of my Tundra pickup. But I tried harder the next time -- i.e. stomping as hard as I could -- and was rewarded with a little ABS actuation just before stopping. And again....and again...each time getting to ABS actuation a little easier and quicker until finally I got an initial chirp from the tires and then an immediate full-ABS pulldown. In a no-hands straight line, too, so I know that all wheels were involved.

Like galley slaves in the old Roman empire, these things have to be beaten regularly in order to work efficiently.

 

Hey ddahlgren I see You live in CT. also . I have a spare power brake booster that I bought for My 1990 Vette that wasn't the problem . You can borrow it if You want to see if it helps . Let Me know if You want it and we can meet some where in between Avon were I live and Mystic were You live .
I would recomend that You get the Vette out today in the rain and slippery conditions and stand on the brakes at 20 to 30 miles an hour at least 4 or 5 times to try and get the ABS to work . With all the rain and leaves on the road today You should get the wheels locking up and a good reason for the ABS to kick in . This is what I had to do with My 1990 Vette . I had a soft spongy pedal that wouldn't lock up the wheels till I pulled this stunt the other day and now they seem to work fine .

 

Wish I spotted this earlier picked up a booster from Napa yesterday and about halfway through changing it and need to finish. My problem is a very hard pedal so hope I am on the right track..
Dave

 

Have you thought about trying to heat up those Hawk pads? You may have to get them nice and warm before they start to grab.

 

If I need to heat them up they are useless as a street pad as advertized. You have no warning when some moron pulls out in front of you... If the booster fixes it I will post results. If not all that is left is the abs and not looking forward to that possibility.

 

Does anyone know the height of the pushrod either above or below the face of the booster? GM FSM says to use gauge J38083 it is a sheetmetal looking piece with a go and nogo height but no dimensions at all.

 

Setting the booster rod clearance to the back of the M/C? The rod goes into a cup in the back of the M/C. I used an L shaped piece of metal that I bent to measure with. I believe I left about 1/16" of a gap between the rod and back of the master cylinder.

 

OK I will give that a try. I have depth mic that I can use and will just make a little sketch and do the math. The FSM said to put 20" of vacuum on the booster while making the measurement but that seemed to change nothing other than making my hand tired from pumping the mighty vac..LOL

 

What I did was get the pushrod to look the same distance as the old booster. Put some dielectric grease on your finger and touch the end of the screw and pull your finger away. That will leave a trail of grease sticking out.

Then bolt the master down, unbolt. And look at the grease to see how close the master is to the screw.

 

Ok my saga continues. Does anyone know any rebuilder that either sells or will rebuild my booster. The Cardone trash is just plain wrong. The first one had studs so short the nuts could not be tightened to the locking section of the nut and the rod that attaches to the pedal is about 3/16 too short dropping the pedal height about 3/4 of an inch almost even with the gas pedal. The second one had longer studs but same short pedal rod. They list the booster their part # 71401 as fitting 85 to 91 and yes it will physically bolt on it is wrong. GM lists the PBR made booster as fitting 90 and 91 only. I am assuming this is because the 90 and 91 had the ecu bracket and gasket between the booster and firewall causing the need for the longer pedal rod. I did find a couple of new old stock parts but they are super pricey to say the least and carry no warrantee.

Some tech I have learned along the way
85 to 89 pedal rod is 6.375 from the booster face to the bottom of the hole the pedal goes through
90 and 91 is 6.6

The distance from the master cylinder rod to the face on the booster is 0.075 below according to Cardone and the measurement on my old one is 0.068

According to GM the booster ratio is 3.5 to 1 so 100 lbs of push on the pedal rod is 350 on the master cylinder rod. If you multiply that by the pedal ratio you can generate some serios pressure. I have also realised the booster can be broken internally yet still hold vacuum so there is more than 1 failure mode.

If anyone has some workable ideas I sure would appreciate it I can not afford new GM at 350 to 650 depending on where you buy and not gutsy enough to buy NOS with no warrantee.
Dave

 

The NOS part might not have a warrantee but, the parts store should have a return policy if the Booster physically does not fit or has a defect.

 

They are being sold as is where is. same part number as factory on booster same tag etc. If non functional too bad sorry about that..
Dave