88 GT brakes. (Page 2/2)
reinhart JAN 27, 03:23 AM
1) Go look at your rear calipers. Are the brake pads in contact with the rotors?
2) If so, then your problem is probably elsewhere: master/slave or air somewhere.
3) If the pads are not contacting, it's telling me your rebuilts are garbage. No rebuilders I know of rebuild the internal piston, not the Fiero store, not calipers online, especially not the local auto store. They change the other seals but not the piston seals since they can't get that seal anymore. You likely wasted your money since they didn't rebuild the piston which is the biggest problem area for 88 rears.

I don't believe new rear pistons available anymore.

I have seen 88 rear piston rebuild kits with the new piston seal for sale on ebay a year or so ago, but they weren't cheap. Not sure if they're still on there.
Tallguy68 JAN 27, 02:02 PM
Thanks for all the info!

As an update, I did The Ogres adjustment over my weekend and been test driving around my neighborhood to test. I’ve got a pretty good little circle that I can do a test that includes freeway and in city and be only a mile or two from a safe port.

So far, the car stops and the parking brake works. Yeah! But….. the brakes don’t lock up. It stops good, and within a safe distance, but I would expect wheels to lock if I stomp hard? I want to put a few more miles testing, then I’ll jack it up again and see what it all looks like, get the missus to pump the brakes, see how they’re working.

The gist I’m getting from all of the rebuild and parts posts is that if the calipers don’t function properly, we’re kind of screwed. So if turns out the calipers are shot, and there are no parts available there’s one more question. One that might need its own thread?

What are the options to get functioning rear brakes? I have read that people have put front calipers on the rear out of frustration, but as this is a 5-speed, I kind of need that parking brake.
theogre JAN 27, 05:06 PM
⚠️ Warning: Do Not put Front calipers on the Rear Axle.
This is 100% Illegal in most states and many other countries. Including most US States w/o inspections or poor inspection that doesn't actually look/test for missing/bad P-brake parts.
Driving a vehicle w/ "Cheater Fixes" and other Illegal Mods Can and often does go very bad after a wreck and you get sued etc.

As to "Locking the wheels" while braking...
Fronts can/will lock up depending on road surface, tires, and more things.
Rears almost Never lock on most pavement types. Can lock on wet roads, sand and other dirt on roads, etc. but harder to lock even then unless you Drive Too Fast for Road Conditions also Illegal w/ big Fines and Points.

GM and most others will try to never lock the rears because can easily spin out and worse.
That's All vehicles without ABS, TrackCon or other computer controlled braking.
And even before Nader "Unsafe at any Speed" crap book.

Is a function of the Proportion Valve (Often part of Combination Valve for cars.) and brake bias covered in my Cave, Brake Upgrade
But 88 Fiero is "unique" in bad ways because have near 0 to No System Bias except the prop valve. Meaning if the Prop Valve fails then is a "ticking time bomb" waiting to spin out w/o warning because Rear Brakes Will Lock at "Random."
Nearly all DOT Legal vehicles from car makers have Bias built into them with Front brakes made way different to the Rear so Prop valve often does nothing for most braking.
sanderson231 FEB 01, 09:25 PM
Back to the e-brake. In order to understand how the e-brake works, one has to understand how the master cylinder works. There are effectively three pistons in the master cylinder; the front brake piston, the rear brake main piston and the rear brake quick take-up piston. The front brake piston is farthest forward, the rear brake main piston is in the middle and the quick take-up piston is in the rear. The first two are in the small diameter of the bore and the quick take-up is in the large diameter of the bore. The front brake piston has a spring and the rear brake main piston has a spring. The front piston spring is weaker than the rear brake piston spring. The rear brake main piston telescopes over the front brake piston. The front brake piston and rear brake main piston have cup seals that seal in one direction but can pass fluid in the other direction similar to a check valve. The quick take-up piston has an rear o-ring that seals in both directions.

When the brake pedal is applied the brake booster presses on the end of the piston causing piston to move forward. The front brake spring compresses and fluid is compressed to start applying the front brakes. There is initially little compression of the rear brake main cylinder spring and therefore little pressure is applied to the rear brake lines. After sufficient pressure builds in the front brake system then the rear main cylinder spring compresses and the rear brake main piston starts applying pressure. The fact that the springs have different are spring rates, I think creates some front/rear brake bias.

Now to the quick take-up piston. This piston communicates with the rear part of the brake reservoir on the master cylinder. There is a small black plastic device in the bottom of the reservoir held in by a circ clip. The best way to think about this device is that it is both a spring loaded pressure control valve ( I cut one apart and there is a spring) in parallel with a check valve. When the master cylinder piston moves forward, fluid flow up through this device and the pressure is controlled to a few psi. Fluid flows past the rear cup seal of the rear brake main piston and a few psi of pressure is applied to the rear brake lines. The geyser of fluid in the rear reservoir that happens when the brake pedal is pushed is the excess fluid being relieved by pressure control valve to regulate the pressure on the rear brake lines. If the calipers are in good shape this is sufficient pressure for the brake pads to lightly contact the rotors. When the master cylinder piston retracts, fluid flows from the back reservoir (the device now acts as a check valve) to fill the space created by the retracting piston. Light pressure is held on the rear brake lines because there is no way for fluid to get past the cup seals for the rear brake main piston. I know this is true because the pressure control valve got plugged or by some other means became blocked. So when the pedal was pushed lots of pressure was applied to the rear brakes and there was no way to relieve it. There was virtually no free play in the pedal. The pedal force was very hard. The car felt like there were no front brakes. The rear brake were locked on and I warped the rear rotors and had a caliper catch fire from a fluid leak in a 7 mile drive. Upon checking there was no geyser in the master cylinder rear reservoir when the pedal was pushed. Replacing the master cylinder solved the problem.

The quick take-up feature of the master cylinder should make the rear brakes self-adjusting. However if the caliper pistons have a tough time moving in the bore then the light pressure applied by the quick take-up cylinder may not get them to move. Then is becomes necessary to manually adjust the emergency brake by taking the lever off and rotating the screw. Pumping the pedal should get the rear pads in light contact with the rotors. If that doesn't happen something is wrong.

Also the rear brake system has to be hydraulically tight for the emergency brakes to work. When the emergency brake lever is rotated the pads will push on the rotor. At the same time the caliper piston is trying to push fluid back to the master cylinder. If there is any air in the line, the air will be compressed and the emergency brake will not hold. If the cup seals for the rear brake main piston in the master cylinder leak, then the e-brake will not hold.

------------------
formerly known as sanderson
1984 Quad 4
1886 SE 2.8L
1988 4.9L Cadillac
1988 3800 Supercharged

theogre FEB 02, 01:23 AM

quote
Originally posted by sanderson231:
In order to understand how the e-brake works, one has to understand how the master cylinder works.

No you don't and rest of that has problems. Mainly you had two or more problems and while Iffy/bad MC can affect the rest, the failure of rear self adjust isn't related.

1. Quick Take-up section affect Both circuits Before each contact and clamps the rotors. Has little to do w/ auto adjust the calipers on either axle. It Can't add Fluid Volume to the Fronts but pushes that section faster when Not @ Working Pressure.
2. The MC rear "geyser" is overflow for QT section. W/o the valve releasing QT Pressure when the function is done, you have a very hard pedal, so hard you often can't stop the car.
See my Cave, Quick Take-up notes with pictures of QT "check valve."

QT MC was originally design for Front Disk w/ High Clearance Calipers. "Minor" changes to Main Seal to allow it to Distort more under hydro load to pull pistons more to give better MPG. (Because Even Tiny Fractions of 1 MPG can affect CAFE Points from US Gov that helps or hurts GM and others Bottom Line to the tune of Hundreds of Millions to Billions of $.) GM used QT MC "backwards" to help "Fiero Type" rears to work "better."


quote
The quick take-up feature of the master cylinder should make the rear brakes self-adjusting.

Maybe but often Not.
4 ref: Front self adjust anytime a piston is push out more then main seal distorts then seal only return to rest and only return the piston a tiny bit. Can happen during QT but needs Full Hydro Pressure to Fully self adjust.

Rear self adjust as shown in Cave and often happens After QT is Done and just Before Main Hydro is @ Full Pressure to stop the car.
WHEN the two pistons fail to do this then have big problems that can cause warp rotors to brake failure and even fires.
Manually Adjust the Rears, even if they seem to work for now, will cause same problems.


quote
Also the rear brake system has to be hydraulically tight for the emergency brakes to work. When the emergency brake lever is rotated the pads will push on the rotor. At the same time the caliper piston is trying to push fluid back to the master cylinder. If there is any air in the line, the air will be compressed and the emergency brake will not hold. If the cup seals for the rear brake main piston in the master cylinder leak, then the e-brake will not hold.

No.
While you push the pedal to help you when applying the P-brakes per Owners Manual in Fiero and some w/ same rear brakes... The P-brakes Do Not use Hydro to Work nor does it put any pressure in hydro side. In Fiero, Equalizer holes built into working bore of MC and QT "Check" Valve stops this from happening.

The screw and "nut" (Orange) pushes Both pistons into the pads by Total Mechanical action via the tapper between inner and outer pistons.


No vehicle is allowed to use Hydro as part or all of P-brake function as directed in FMVSS 571.105 and 571.135 for newer cars, after ~ 95 model year. GM w/ NHTSA "Blessing" made false statements for a brake recall that blame owners for don't using P-brake to adjust the system etc just made more problem in the end.

While DOT FMVSS only test P-brake as P-brakes for a Parked Vehicle... If Fiero and many other hand or foot operated P-brake works, You can use them as "Emergency Brakes" to Stop a car if the Hydro has completely failed. Will take a lot longer like shown in many owners manuals but will stop you. Unlike nearly all Electric P-brakes on new cars that won't activate when vehicle is moving, have electric problems, etc.

Is Also Why can't use "Line Locks" as P-brakes because Hydro Parts Will Leak down enough pressure to let a car move and often hit things or people. This is 100% Illegal in many places and even if "legal" for given State, many Car Insurance Co can void the policy for this crap and/or expect to get sued after a wreck.

[This message has been edited by theogre (edited 02-02-2023).]