ddahlgren

This has kicked my butt to the point I parked the car for a year to end the frustration. First lesson a 90-91 booster is all by themselves to the point the dealer needs a VIN number to provide the correct one if they had it or a rebuilt. Cardone and others I have found lump the 91 with all the other C4s that use a different booster.

Sent mine out to be rebuilt and no better. The pedal is hard and car does not stop like it should. Things that were done all about 1 1/2 years ago. Rebuilt front calipers and upgraded to 13 inch brakes, Napa rebuilt rear calipers, NAPA rebuilt master with DRM bias spring, Earl's stainless brake hoses, flushed brake lines with fresh fluid before connecting hoses to calipers. The rotors are all NOS factory ones with Hawk brake pads.

I seem to have good vacuum at the hose going into the booster. I am at a bit of a loss here.

Brake experts how would you check this out?

jv9999

If you pull the check valve off the booster (after shutting the engine off) do you hear air rushing in with a pop? Do the brakes work other than being hard?

gerardvg

Hi

Have you checked the rod from the booster to the master cylinder?
it is adjustable and if it is adjusted wrong it could be your problem, hopefully someone can post the correct dimensions of that rod for the 91.

Can you feel a pedal change from no vacuum in booster, with engine off depress the pedal a few times. Then start the car and the pedal should go softer, make sure you have good vacuum pull the hose off and start the car and it should hold your finger down.
The hose gets its vacuum from the inlet manifold on drivers side near the distributor.

Even with a blown engine i have never run out of vacuum for the brakes, perhaps replace the hose from inlet manifold to booster in case it has cracks affecting vacuum to the booster.

Another test is just run the engine and turn it off, wait 30 seconds or so and pull the vacuum fitting on the booster a little to one side it should hiss.

Usually the diaphragm tears and you can hear a hiss from inside the car when pressing the brake pedal.

I have the J55 13 inch rotors and calipers on my c4, they work great comparable to new cars. very little effort required even at very high speed repeated applications with the hawk pads.

Several on this forum have also mentioned the ABS pump sometimes needs to be triggered a few times to bleed the pump, just bleeding the calipers does not bleed or prime the pump fully.

Good luck, hope you are back to braking with confidence soon.

L.V. Vette

I've had hard brake problems with my 91 since I purchased it last year. Previous owner replaced everything from pads to booster. Mine also feels like I have manual brakes. Sometimes it's scary driving like that. Interesting to see what all the suggestions will be

Hoser

Same here on my 94. Been fighting hard brake pedal but (scary) poor braking for 3 or 4 months. Wore out my tech 1 looking for codes and testing system. Had it beat for a while after a booster, M cyl(2 of them), hoses, gallons of brake fluid through it and EBC yellows on front. Then, while sitting in a p lot, foot on brake, in park, pedal starts to come up (by itself) and bam no brakes. Tech 1 again, nothing. Tested the booster and the new one won't hold vacuum, so either the diaphragm or valve let go. That was an A Zone metal replacement. Got a AC Delco on order. Will see what happens.

L.V. Vette

Please update after new unit. Thanks

Hoser

Will do..

Hoser

Well the new booster from AC Delco restored the brakes to "they won't kill you" level. Didn't pull the seat this time but wasn't too bad...getting good at this now. But installing the inboard nut that holds the booster to the F wall is still a time consuming pain as my back will attest. AZ unit had a leak as my " my Home Depot fabbed tester" suspected. Anyone know if the ZR 1 dual diaphragm tester might be a solution (does it even bolt up) or why they went that way?

RWDsmoke

I went through all that years ago. I replaced the booster, master cylinder, c5 brake conversion with all new GM parts, brake hoses, still hard pedal and lousy brakes. One day out of frustration I removed the ABS fuse and while driving down a deserted back road I stomped the brake pedal with both feet with all my weight thinking I would probably blow something out. All four wheels locked and when the tire smoke cleared I had great brakes. Still work great many years later. I never found out what it was but it is fixed. Best guess, something was stuck in the ABS. No codes were present and ABS appeared to work normally if you pushed hard enough on the pedal.

bosshog8

Bad ABS. I recently bought my 90 with hard pedal, pulling, terrible stopping, etc. The ABS was internally faulty and unplugging it did nothing. I removed it to get the brakes to return to normal.

Hoser

Can you share the details?
Have often entertained that same thought.
Did you run new lines? Or just fab connectors at the valve body?

bosshog8

Sure, you need to joint the rear in to rear out with a bubble flare coupling. I forget what size (probably 10mm like the front) but its for sale in the local auto part store. Then you need this bubble flare t , https://www.appletreeauto.com/METRIC...E-113-611-755/, to connect the front in to 2 front outs. There is plenty of line to manipulate and get the fittings on. Also, FYI, no proportioning is done at the ABS. It's all in the master cylinder so no need to do anything else to complete the removal.

Hoser

How did you ever find those fittings?
Excellent!
Tanx mucho!

ddahlgren

Ok finally did a quick read in the FSM under brakes general hard brake pedal. They had checks for all the vacuum parts like lines check valve etc. and glazed pads and rotors causing a hard brake. the rest were all related to booster internally so new booster time if not lines or pads/rotors. Will check the lines and if ok get a set of cheap soft pads scuff up rotors etc. I that does not fix it the booster comes out to go back to the rebuilder.

There was no mention at all about ABS causing a hard brake pedal.

ddahlgren

Ok did a quick test, engine running pried back on the booster check / grommet and engine was not happy with that I guess from the air leak. Let it run for a few minutes and waited a full minute and could hear a hiss prying back on the check valve. This tells me the booster gets and holds vacuum. In the factory they listed other things that can go wrong with the booster and have nothing to do with diaphragm so I suppose it can hold vacuum and still be defective. They did list bad glazed rotors can cause a hard pedal but sure seems like a reach to me and don't see how that can affect the pedal feel on the booster just have poor stopping distance in my mind same with bad ABS. How can that affect pedal feel? Long stopping distance I could believe. I can get the dealer to hook up to the ABS get codes and possibly cycle it but a pricey test. I hate to go through a pad change that in my mind has nothing to do with the pedal. I don't see how the master cycler could do it but all bets are off there.

Things done to brakes
switch to 13 inch C4 fronts
Rebuild front calipers
Rebuilt rear calipers from NAPA
DRM bias spring in MC
Stainless brake flex lines on al 4 corners
Bleed brakes until clan fluid came out

OK I am up for what on the possible list makes the most sense. My current vote is the booster but a pain to change send out to rebuild shop to be checked or done again. I would have to hire someone to help with that given my health issues.

bosshog8

I’m not trying to be a jerk or sound like a “know it all” but you have 2 separate replies with empirical evidence as to the likelihood of your problem and your going to start replacing things twice to try and remedy the same problem the first time didn't fix?

I'll help you out with "How can that (ABS) affect pedal feel?" The ABS has the ability to completely shut off all brake pressure to the wheels while you stand as hard as you can on the pedal. It can affect every feedback sensation you get from the brakes.

FWIW - Hydraulic brake systems are incredibly simple, reliable and work very well (power assisted or manual).......until you put an ABS in the middle. Its an unserviceable, unrepairable, unrebuildable achilles heel of the brake system that on more than 1 occasion "couldn't possibly be causing my problem".

ddahlgren

Not sure on the multiple theories but did list everything done so they can probably discounted and not waste anyone's time asking if they have been done. I did have a long chat with the booster rebuilding shop in Oregon and their thought was something got missed in the testing before shipping. They are happy to test it and go through it no charge as needed. What I don't want to do is replace parts twice and this did start out as a complete brake system rebuild so not blindly throwing parts but admitting they are 25 years old and steering and stopping a must have.

What keeps me thinking booster is the pedal does not take up correctly. It feels the same engine running or off. It was suggested it did not have enough vacuum but running the engine around 3000 rpm made no difference. It goes from 1/8 to 3/16 inch free play to rock solid with no initial take-up that is light then gets stiffer. The rebuilder agreed with this saying if the is something wrong beyond the booster it will not stop correctly but should take up smooth getting stiffer as the pedal is pushed more. The booster comes out in under an hour and can get boxed up and sent back to check for the cost of UPS. I can't see any more logical way to approach this.

bosshog8

That may be a reasonable approach. If you find yourself in the same situation after the rebuild I would absolutely skip the dealer as they will gladly take your money and solve nothing. It costs about $20 in fittings to remove the ABS.

Tom400CFI

I just did it on lay '89 'Vette Kart for ~$11.00.

There have been a handful of threads on this forum where folks "discovered" that the ABS was causing a hard pedal and poor braking. They "fixed it" by invoking ABS function by hammering the brake pedal on loose gravel several times. Since that costs nothing, I'd do that first.

ddahlgren

I think taking it one simple step a time is the smart move. If I had one a pressure gage on the master cylinder would tell a bunch.