So a few weeks back the brakes on my cousin's 1990 TII failed on him. The pedal would go straight to the floor and no brakes. There was no loss of fluid and while pumping the brakes you could hear air in the master cylinder. We ordered a rebuilt BMC unit from Malloy Mazda but unfortunately they sent us the wrong one (not the turbo BMC).

We decided to bench bleed the faulty one to see if maybe thats all it needed. We were able to bench bleed it no problem and got rid of all the air bubbles. Installed it back into the car, went to bleed the brakes starting at the right rear... nothing. No matter how much we bled, we were getting nothing out of the rear brakes. Decided to try the fronts, and they had plenty of pressure and bled out great.

Attempted to bleed at the ABS unit... same problem. I was getting fluid out of the front bleeder hole, but not the rear. Next tried removing the lines off the proportioning valve (top lines between ABS and PV) and replacing them with the bench bleed kit lines routed back into the BMC resivour. Again, more of the same, front flowed through and nothing from the rear. Finally, tried the same thing but before the PV and sure enough we get good fluid flow out of both lines.

I know the proportioning valve is supposed to restrict the flow of fluid to the rear brakes, however it still should let it past. I was able to physically blow through the rear passageway (with the valve removed from the car) so I do not believe it is clogged. Once we bench bled the current BMC it felt fine. We can even drive the car as is and it stops fine (only using front brakes) with good pedal feel.

Is it the BMC not creating enough pressure to force fluid through the PV? Yet is seems to be holding pressure to the front brakes and pedal feel is good so far. Could it possibly be an issue with the proportioning valve creating too much of a restriction.

We were hoping to get the car on the road by next weekend for a big meet, but by the time the BMC gets sent back and they send us a new one that may not happen :-(

 

I'd try a different proportioning valve.
At low applied pressure the valve doesn't restrict flow to the rear circuit at all, restriction begins and increases as pressure increases.

Can't explain why you could blow through the valve with no apparent obstruction, yet fluid won't pass.

Kinda easier to replace the valve rather than try to "think" it fixed...