Brake issues - possibly proportioning valve? (Page 1/1)
symbol-soup NOV 10, 06:20 AM
Hi All,

New UK member here after having just bought a 1986 SE 308 (not a mera) look-a-like that is right hand drive.

I've found numerous issues with the car and solved a few of them via the forum, it's a great repository.

I've had an issue with the brakes. They were really spongy and not stopping the car. I've had the callipers, lines, pads and master cylinder replaced by my local garage but he is struggling to get the brakes working. If he blanks off the lines from the master cylinder the pedal is rock solid (but obv no brakes) so the MC is good.

The only thing not changed in the braking system is the proportioning valve, but I've read that these rarely fail. What would the symptoms be of this failing and does anyone know where I can buy one. The usual searches ebay have brought up nothing. I have to order my parts from the US so before and I go and by more parts, I thought I'd ask here first.

Thanks in advance.
Cliff Pennock NOV 10, 06:48 AM
When you say it's not stopping the car, do you mean it doesn't brake at all or it brakes very poorly?
olejoedad NOV 10, 09:25 AM
Spongy brakes indicates there is still air in the lines somewhere.
Be very sure all fittings are very tight.
Be very sure that all calipers have the bleeder ports UP, and that the rear calipers are properly adjusted so the the brake pads are in slight contact with the rotors when the parking brake is connected, but not engaged.

Use a vacuum bleeder.
Order of bleeding is left rear, right rear, right front, left front.

In twenty years of working on Fieros, I have never seen a bad bias valve.
theogre NOV 10, 12:31 PM
The Combi/prop Valve and MC can get Crushed ports stopping flow to calipers.
Is cause by fools that over tighten the steel nuts and lines into the aluminium bodies.

Can See some of this damage in my Cave, Combination Valve where some ports are nearly crush close where the steel flare end hits.

Otherwise Combi Valves rarely go bad. Can get crap in them blocking fluid flow or cause binding of moving parts.
Only GM Part made Combi Valve available via Dealer Parts but stopped making/carrying most Fiero parts Decades ago.
Getting used combi and some other brake parts is iffy at best because often means this part was stored out of a "sealed" system and brake fluid sucks water out of air then "rots" the guts.

Spongy Pedal is not a Combi problem.
Most often you have air in the lines, calipers or MC.
MC often needs "Bench Bleeding" if replace or just tank runs dry doing other work... See my Cave, Bleeding MC notes
Might get MC air out if you jack rear of car to level out the MC or tip MC front a bit down. But you need to jack the rear of car high and that can be dangerous in several ways...

Low Pedal is not that problem too.
Low Pedal in Fiero is often cause by rear calipers with iffy pistons that won't self adjust. See my Cave, Rear Brakes & Rear Piston notes

I list spongy and low because can have both problems or confuse the two terms and isn't = to say.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

Kitskaboodle NOV 10, 11:17 PM
It just so happens that a few months ago my proportioning valve needed to be replaced. I believe what happened is that I blew a seal in my master cylinder while driving. When the proportioning valve sensed a major brake pressure drop in one circuit (front or rear) the proportioning valve piston automatically shuts off the circuit that has this pressure drop so that you can at least use the other circuit to stop the car. After you fix the brake problem, you must reset the piston into the center portion of the valve. Until you do this, the valve will trigger your brake light on the dash to stay on. I was not able to center mine back into position. I ended up getting another valve from an 86 GT in a PNP.
Kit
fieroguru NOV 11, 06:48 AM

quote
Originally posted by olejoedad:
that the rear calipers are properly adjusted so the the brake pads are in slight contact with the rotors when the parking brake is connected, but not engaged.



X2

I suspect there is a large gap between the rear pads and the rotor and that the parking brake piston takeup needs to be adjusted. When there is a gap, as the press the pedal, you will not have any brakes until the rear pads make contact with the rotor. This will cause a soft pedal and long travel before any braking and even once the brakes start, braking is significantly reduced. While this issue isn't isolated to the FIero, it is easy for mechanics not familial with Fieros to overlook, and is especially common when rear brake pads or calipers have been replaced.
sanderson231 DEC 08, 05:33 PM

quote
Originally posted by symbol-soup:
If he blanks off the lines from the master cylinder the pedal is rock solid (but obv no brakes) so the MC is good.



With the master blanked off there should still be a couple of inches of free play in the pedal before it gets firm. If there is no free play it is an indication that the spring loaded pressure relief valve in the back of the master cylinder is plugged. I had this happen and the pedal was rock hard with no free play. The rear brakes were applied all the time and the car stopped like there were no front brakes. Because the rear brakes were locked on I ruined the rotors and calipers in a 7 mile drive (leaking brake fluid actually caught on fire). To check the functioning of the pressure relief remove the cover from the master cylinder and have an assistant press on the brake pedal. There should be a geyser of liquid spurting upward back reservoir of the master cylinder. If there is no geyser then the master cylinder is bad.

That said. I could not get the brakes on this car (88GT with Lebaron brake upgrade) to bleed with either the pedal method or vacuum applied at caliper. I bit the bullet and bought my own pressure bleeder and that worked like charm.


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

[This message has been edited by sanderson231 (edited 12-08-2022).]