Anyone have any suggestions?

 

The purpose of a proportioning valve is to reduce the brake pressure going to the rear wheels.

As the car's weight shifts forward, more traction is available in front and less traction is available in the rear. The proportioning valve is supposed to be calibrated to proportion the brake pressure roughly equivalent to the weight transfer.

However there is additional proportioning built in by the manufacturer for safety. If the rear wheels lock up before the front wheels, you will suddenly have much less brake force in the rear than the front. This will tend to spin the rear of the car around to the front. The manufacturers all design their proportioning valves so that the rear wheels will never lock before the front wheels. This usually works out so that the rear wheels never lock at all except on ice.

The proportioning valve is not stuck. If it was stuck, your rear wheels would lock up, just like when you pull the parking brake hard whithout using the front brakes. The question now is, "Is there something wrong with your rear brakes or are they working at a correctly reduced level?"

ed

 

I believe the prop valve is bad, perhaps its sticking internally and dumping the rear brake pressure from the MC. Today, I rebled the MC, I get very good fluid flow from both outlet ports. I also checked the brake booster, and it checks good. I have a prop valve on the way and will update this post after its installed.

 

Anyone else?
I have confirmed my identical problem with another MK1 owner. His fix was to replace the prop valve. I would think that others would have run into the same problem.

 

sorry man. All my cars use Wilwood prop valves.

 

I have seen the pictures. Maybe I will go to the wilwood valve in the future, for now I just want my SC back on the road.

 

Swapped in a prop valve- same problem. i am going to replace the recently installed master cylinder. More on this later.

 

anybody had this problem and found out what to do?
My rear brakes won't bleed.

 

Pull the line from the rear caliper.
Does fluid come out? If yes then your caliper is bad, if no than move to next step.
Attach pressure bleed to line and see if hard lines are obstructed. If yes then replace or clear, if no than check MC.

 

it was the mastercylinder. I replaced the brake proportioning valve and checked to see if fluid could be pumped out from the line going into it and it was just bubbling. Took the mastercylinder from my wrecked car and it worked, bled the system and now i have brakes.

 

Cars run two brake circuits. On MR2's, there is a front circuit and a rear circuit.

Basically your rear channel was not functioning.

Common problem is too much air in the master and therefore not pushing enough fluid to actually get the bleeding procedure going. Typically problem is not bench bleeding the master prior to installing it.

Easy fix, crack open the line to the rear circuit at the master cylinder. Pump the master some to get some fluid cycling out. Once that occurs, tighten line to rear circuit and try bleeding again. Good luck!