ddahlgren

Ok last in my list of questions.
91 couple upgraded to 13" front rotors new MC DR bias spring Hawk HPS pads. Car will not stop without serious brake effort. Booster rebuilt as a 91 booster is a 1 or 1 year application and nothing aftermarket available that is correct. Done by a pro shop and tested before shipping.

I have 2 gut feelings as pedal feels right. ABS has issues, MC is defective or brake pads just wrong. No one around here can run diags on ABS so no way to know if that works. I did set it off once going around a corner in the rain at 30 when a car pulled out in front of me. Dry road never going to happen. Bottom line it does not feel safe and my 2002 Pontiac can easily out stop it and get full confidence.

Solutions
Are pads just wrong?
Booster 1 year only and all I found was GM new at 600+ the answer no there.
Car has new NAPA MC anyway to check it? Bleed brakes to death following service manual procedure.

Suggestion just bypass ABS but how? Are pads all wrong? Is MC trash though new? Is DRM bias spring trash?

Open for thoughts..

s carter

Sometimes new pads need to be kind of burned in to get good braking but if your pads have been in for a while they are probably past that point by now and should be good now.

how is the pedal is it super high well bled and when car is started pedal drops a moderate amount, also do you have a good strong Vacuum at the Booster.

dizwiz24

had some issues on my93 where id get abs activation in the dry under mild braking sometimes .

turned out that the rear abs wheel speed sensors had tons of metal filings attached to the sensor surface (seemed magnetized)

cleaning them off solved the problem

gerardvg

Hi

I have upgraded to the 13'' rotors on my 85 vette, I have great brakes requiring very little effort to lock the wheels.
There has to be a problem, Hawk HPS plus brake pads are the best for street and track use a high friction pad that should bite very easily.
Even Hawk street brake pads are really good, I suspect you have an issue with air in the brake fluid or trapped in the ABS.

Do a couple of tests and really brake hard to activate the ABS to get the air trapped out, the bias spring is to get the rear brakes to work harder.

Before starting the car does the pedal feel firm of soft?
Start the car and he pedal should go down a little.

Note I have found bleeding the brakes the old fashioned was by pumping the pedal tends to tear the booster diaphragm, I only gravity bleed my brakes now. Just remove bleeder screws and refit loosely after 20 seconds I tighten the bleeder screws, I have a hard pedal minimal travel and I feel my brakes are better than my modern company vehicle.

Check your vacuum hose are fitted to booster and have no cracks, did they bleed the master cylinder before fitting to car?

The most important is the correct booster pushrod adjustment, the booster has most effect at a certain travel. If the rod is not adjusted correctly it could be your problem, I would check how much pedal travel till the pedal is hard. I have very little pedal travel, see the link to booster rod adjustment below.

Now is the master cylinder hole the pushrod goes into at the correct depth, lots to think about and to check.

Good luck it should be something simple

https://www.google.com.au/search?q=c...G32mKJShPuM%3A

TorchTarga94

There was a thread sometime back about a hard pedal and not very good braking performance. The solution (IIRC) was to exercise the ABS system. Find a long straight road and get the car up to 60MPH or so and really lay into the brakes as hard as you can. Do not let the car come to a full stop, repeat this same procedure several more times (4-5 times) and then see how the brakes feel. I tried this on my 94' and noticed my braking performance seemed much better afterward. I need to do it a little more.

C4vettrn

Did you follow the FSM for bleeding. The ABS/ traction control system requires a special bleeding procedure and there is a bleeder valve on the pump i.e. in compartment behind drivers seat.

maj75

I have the big brakes on my '94 Z07. I just replaced all the brake lines, calipers and rotors and added the DRM bias spring. I also have Hawk HT-10 pads as this car was built with track use as the emphasis. Bleeding the brakes was a process as every line was new and the system started with absolutely no fluid in the lines. That said, these brakes are awesome, the best I have ever used and that includes Boxster S and M3. Effort was reasonable, I could brake to the point of ABS engagement. ABS activation was mild.

Are you sure the booster is good? I'd make sure that is working correctly and then rebelled the brakes. Drive the car on some grass to activate the ABS pump. Open the hatch and prop it open and you should have no problem hearing it.

antfarmer2

Pump up your brakes and hold the pedal and start the car. The pedal should go down a bit. If it does not might need a new booster.

l98tpi

Here is my suggestion; get rid of the bias spring, gravity bleed the brakes, and you don't need a special bleeder on the abs unit. I have been gravity bleeding my brakes on my 91 for 8 plus years and only gravity bleed. And as far as the Hawk HPS, they are good pads for street cars. I run them on my street 96 and they work great.

ddahlgren

No need to pump them engine off pedal moves a short amount and stiff, start car goes down maybe 3/8 to 1/2 inch and gets a tad softer. Rebuilt here front calipers napa rears and napa MC with DRM spring. Hawk HPS pads bleed using FSM instructions. I doubt if any air as the brakes right there no lights on indicating any problem. 13" front rotors 12" rear GM NOS. If it stopped as good as my 4 door 2002 Pontiac I would be happy. Booster is rebuilt and tested by a rebuilder I trust as he identified brake fluid getting into it causing problems and the reason for the napa MC all work was done at the same time while apart. Different pads? I want instant on and don't race the car but want to stop or slow want it now. The old Pontiac can toss your head into the windshield if no belts on and set off ABS on 245/60-16 Goodyears that are fresh on a dry road.

cardo0

ABS is all rear braking and most of your braking is done by the front brakes. It kinda reads like a M/C problem like u say or a proportioning problem. Does it stop straight when you use max brake? If only one caliper good/bad it would turn.

Just my two cents here but the first thing I upgrade on brakes is install SS braided flex line as they expand less than stock flex lines and conserve pressure.

Maybe a call to one of the brake vendors could help.

Good luck.

ddahlgren

Got the SS all 4 corners. yes stops straight just not briskly.

ddahlgren

Talked to the guys that rebuilt the booster. Told them per their advice installed a new MC when booster reinstalled. Discussed the pedal feel and they said check vacuum again and check valve again. One of the things they suggested was checking brake system pressure. Tried several dealers and local repair shops auto zone and napa for rental tools to no avail there either. I had the opportunity last week to try the brakes in a half dozen cars same vintage and all feel identical and nothing like mine so not imagining it. If anyone knows someone in SE CT shoreline area or western RI shoreline area that can make this check a tip would be appreciated. Basically the pedal goes from high and hard key off engine off to 1/2 to 3/4 inch lower and still hard engine running. The car sits hate driving it and a death trap as is.

cardo0

Well a brake press guage will set u back $50 on amazon but i couldnt tell u what is a good reading myself: https://www.amazon.com/SSBC-A1704-Br...ressure+tester.

Something i question is if u have a fairly large cam/duration and idle vacuum is low. Possible brake booster isnt getting enough vacuum? U can connect a "Mighty Vac" hand pump just to read the gauge. Really need 12" Hg vac though many say less will work. Easy to measure but difficult to cure as low vacuum will need an electric powered source rather than a vac can. Vac can would increase magnitude of vac only but not volume/quantity.

I think i would make the effort to R&R and bench bleed that M/C again for piece of mind - i dont trust a in-car bleed of M/C due to angle. And im suspect of someone elses work rebuilding a M/C unless they are a brakes only shop.

Finally something we all hate to admit is the "brand new brake pads" have been contaminated. I was in denial on my camaro Hawk semi-metallic pads because of cost but they rapidly went from awesome to lame after upgrading the flex lines. I guess what im saying is after im done with my final bleeding operation i would remove the pads and clean the rotors with cleaner then just install some cheap organic pads and check the results.

Hope this helps.

Joe C

FWIW, I had the same issues with my (first) 90. when I bought the car, and before I did any brake work, I could not engage the ABS system at all, and my braking quality was extremely poor. brakes were so bad, it was unsafe at any speed. after I bought the car, I replaced the MC, installed stainless steel brake lines, tried several different sets of pads, new rotors, bleed the brakes (twice), even took it to a dealer for a brake fluid, power bleed, and ABS diagnostics. same results - poor braking. I did get it to the point where I could get the ABS to engage, but only after standing on the pedal with extreme force. there was no way in hell I could lock up the tires. I did suspect there may be an issue with the power booster, but ended up selling the car before resolving the problem. I have read on the forum about other C4's with the same brake problem. just curious, and this may not have anything to do with it, but have you put a gauge on to read engine vacuum?

good luck with solving this problem. keep us informed - I for one would like to know the fix.

ddahlgren

Stock L98 so is what it is but runs fine. Bench bled MC wore nitrile gloves while doing pads and cleaned rotor with brake clean very fully. I really think pedal feel describes the problem. Booster rebuilder said around 1100 to 1300 psi.

ddahlgren

cardo0 going to give that gauge a try. Without knowing the pressure I think I could chase my tail for a very long time throwing parts at it. It is a 2 person job and avoids a dozen other checks if pressure is good. Trying new pads and cleaning scuffing rotors is a couple of hundred bucks and a few hours for potentially nothing. Playing with vacuum gages and check valves is easy though suspect busy work as well until prove otherwise. The thought of pulling the booster is more than painful at the least. The rebuilder talks about it like it is held in with Velcro. And for a second time yuk. If I do will send MC with it and have them test there until pressure is correct for a given vacuum. If pressure is good have to assume ABS booster lines and MC are all fine. I would think if front PSI good and rear bad ABS MC or lines. If all low ABS MC Booster or lines as long as vacuum is good. Am I missing something here? 50 bucks to cut to the chase seems cheap enough when no one around here at a shop can do it and no store has one to rent. Ready for hints.

cardo0

I really don't know a thing 'bout ABS and really should do more research on ABS. But the early ABS wasn't to problematic until they became more complicated/integrated/computer controlled by about '95, '96 maybe '97.

But verifying pressure at the caliper should tell you where to look next.

I like your technique: diagnose more and buy less parts.

ddahlgren

After a while old age teaches you to be a bit lazy and frugal LOL.