Rear bias valve question

[Stvstr]

I am trying a 5/8" master cylinder on my '88 325is 5.0 swap. It has the Massive booster delete kit. I wanted to see how my rear brakes were working, so I plumbed only the rears into the m/c. I was surprised to find almost no braking power at all and couldn't invoke the ABS.

Switched the m/c to the fronts only and they worked as expected with normal braking power and could invoke the ABS. So I removed the bias valve (below the m/c, against the frame rail) and replaced it with a straight union, connected only the rears to the m/c and they worked fine. Good power and could lock them for a moment before the ABS kicked in.

Then I took apart the bias valve looking for obvious problems, found none, cleaned it out, put it back in and the rears barely work again. Does anyone know if the bias valves fail, or how to test them? Or if they are meant to restrict the pressure to the point that the rears do very little braking? I'd like a better understanding of this before I order parts, because everything in Canuckistan is twice as expensive as in the US.

[jalopi]

I think the stock bias is supposed to be 60/40 or 70/30

Not sure how bad they are but if the rear brakes are hardly working then yeah you've got a problem somewhere. How do the brakes work when you have all four calipers hooked up?

But if my bias valve went bad, I'd get an aftermarket adjustable one instead of another stocker


** just noticed you have a 5/8" master with a massive delete. these really don't push enough fluid to work with our cars, or at least that's a problem that alot of people have run into. you might have to order up a 3/4" master.

[Stvstr]

I have heard that the 5/8" m/c can't push enough fluid, so I am experimenting with a Volvo step-bore m/c. It's designed to use the larger bore to push a volume of fluid quickly then transition to the 5/8" to create the necessary pressure. I have some fittings on order to hook it up, but while I am waiting for shipping I thought I'd try it on the rears only.

I guess I had better order an adjustable proportioning valve to replace mine, as I have no idea if it's working properly.