91 LS400 no power to rear brake!!!

[rwdfreak]

Here’s the story. The car ran fine until one day I experienced partial 4 wheel lock while cruising at 60mph. I stopped at the next exit watched all 4 corner and saw some smoke coming from the rims. So I went slowly back to my house and parked the car for the day. When I came back, the car was acting normally and continued this way for few weeks. Then my uncle experienced the same thing again while driving in his town. My father drove there, unplugged the battery terminal to reset the ECU and the car ran good for a while.
Then it started again. My father called a Toyota mechanics we know and he told us to bleed the brake. So we took a vacuum system and started, but it was not fast so we decide to try the conventional way (pump the pedal while someone is screwing/unscrewing the zerk between each pump) and the engine stalled and didn’t started until we did the reset procedure (8 pumps in 3 second to reset the trac & abs computer). We finished the bleeding and the car ran normally.

Then couple of months after the pedal started to go to the floor intermittently between each braking. Sometimes the brake pedal was high some times it was very low and only the front brake had power. My father had to pump to keep a normal brake pressure. So he changed the master cylinder inner piston that had the rear brake seal worn and that was part of the problem.
After putting back everything, you can still slowly floor the brake pedal while pushing on it and now the rear brakes have no stopping power transferred by the system. The Front brake and ABS are working correctly. No oil is leaking, the brake fluid reservoir is full. Both rear caliper are moving freely, there's enough brake pad material left. You can depress the pedal and see the piston move when it's not clamped on , but it doesn't move fast .

My assumption is that, either we have a giant air bubble in the system, the TRAC system is out of function (although I did tried to brake with the TRAC off) or something is wrong in the ABS mechanism. Can it be the proportioning valve jammed closing the rear brake oil travel? Or as there is a oil cutting valve in the rear oil brake circuit this valve can be closed by the brake ECU. I did the Lexus diagnosis procedure and there is no bad code and I did the reset as recommended by Lexus shop manual.
If you need to see the brake oil and electric circuit diagram we can send you a jpeg picture of that circuit from the shop manual LEXUS LS 400 1991.

Anyone have an idea?

Thanks

 

[rwdfreak]

Still having problem. Bump

 

[Zuffen]

I would lean toward a delaminating rear brake line.

THe flexible lines can perish internally and bits start coming off and block the flow of oil.

I once had one sieze the rear caliper on and it wore away the fricion lining and eventually destroyed the disc.

I removed the caliper and drove a long way home and only lost a minute amount of fluid as the hose was so knackered.

 

[rwdfreak]

I will check that Zuffen.

We removed the rear brake line on the proportionning valve and we couldn't get any pressure. I checked and rechecked the FSM and it see like something is closed there. there is an antileak on the front brake, but not on the rear one. Do this thing need a pressure feedback?

We'll try to get the proportionning valve off the car and clean it.

thanks