Car:1990 Olds 88 Royale
Symptom: Car pulls side to side when braking. I notice it above 20mph. It pulls to the left under light pedal and back to right under heavy pedal. When coming to a stop at speed, it goes left, and then right through pedal depression

What I have done to solve this so far:
- Moved tires around to eliminate heavy torque steer to right - was really bad and a worn tire amplified the problem
- Replaced calipers and rotors as well as wheel cylinders, drums and shoes - bled the system thoroughly util the fluid was clear and free of air bubbles
- Lower ball joints replaced
- new control arm bushings
- just got an alignment

other details:
- in the past year: replaced axles and struts.
- Replace the engine and transmission mounts (the new left front trans mount failed and was replaced yet again.

The tie rods are OK...

I started to replace the master cylinder, but it seems the hard brake line fittings are really stuck and the soft metal on the fittings are already rounded over a bit from previous work.

Questions:
What do the proportioning valves do? Has anyone experience with their failure? What are the symptoms?

Ideas? This really is the only problem left after a nearly complete overhaul of the car.

Thanks!

The prop valves reduce pressure downstream. If they fail, they would either allow full pressure or cutoff pressure all together.

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WHITE WHINE - 1992 SSE Supercharged 236.26 ci (.040 Over) 15.090 at 90.2 MPH on old engine w/ slipping trans & melted O2 sensor - Gen 3 M62 and matching TB, Gen 2 Pully, Zillamotorsports Ported LIM, YT 1.72 Roller Rockers, SII FPR & Injectors, Hypertech Thermomaster chip w/ 160 Thermo, TransGo Shift Kit, Infinity/Pioneer Speakers & a 10" Alpine Type R Sub, all the watts, 140 amp Alternator, Ricepipe CAI w/ heatshield, Pilot Angel Eye Foglights, Clear Corners, '02 17" Chrome Bent 5's, Magnaflow F-Body Muffler and Hi-flo Cat, Ceramic Coated Ported Exhaust Manifolds, Fan Override, Monroe Reflex struts, red calipers
2009 G8 GT - Sport Red Metallic, loaded, SOLO Axlebacks, Rotofab Intake, Tuned, autodim mirror, removed intake manifold cover, HSV GTS triple gauge pod, two tone red-hot shifter and HSV SuperSport steering wheel, GXP rear sway bar and diffuser, 3.45 diff and various Camaro suspension bits, LED Taillights

I also believe the proportioning valve controls pressure front to back.

Does this car have ABS? Why was one tire worn really bad?

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~Randall~


1993 Buick Park Avenue - 197k - Some odds and ends done - Simply won't die
1994 Buick Regal - 78k - Bone stock - Always ready for a good kicking
1990 Oldsmobile 88 - Gone to a better place

I bought these tires from a used tire place. I think they may be factory seconds. Also, they suffered the old struts and alignment issues before those things were corrected. Also, I was interested to read in the factory manual that the master cylinder is "diagonally split" so the right rear and front left tires are on the same circuit.

Have you tried to fully bleed each caliper and drum cylinders?
Do you hear any squeeking when braking?
Have you checked your wheel bearings?

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1991 Red/Maroon Bonneville LE - Was pristine, rebuilding due to deer :(
1991 Black Bonneville SE - Parting out
Maybe someday an SSEi

Problem solved: Mechanic that installed lower ball joints didn't tighten the bolts. Ugh... Now the car is at another shop getting a camber kit and all new subframe bushings. Should be perfect after that.