Last night we put my brake pressure gauges on Rich’s S52 e30. The behavior on his car was precisely the reverse of mine. That is to say, front pressure went nice and high, rear pressure stabilized around 900-1100psi and didn’t want to go higher. We were on the low end of that with a single pedal push, but if we pumped up the pedal a bit we got on the high end of that. Obviously, due to the bias valve, this is the kind of behavior one would expect.
I checked a brake pressure chart from the Service Manual and it says that when the fronts are at ~2000psi the rears should be half that. Since my gauges max out at about 2000psi, I’d say that the #'s work out perfectly. Well, for Rich they do, anyways. For me they’re perfectly backwards.
This kind of explains the idea of the lower brake pressure reaching a plateau. I’ve been reluctant to push my gauges or the hoses too hard so I’ve only been testing to 2000-2500psi.
The obvious conclusion is that my brake hydraulics are backwards. Except that I’ve checked that over and over again. Also, I’ve bypassed my bias valve and the behavior didn’t change. If the bias valve is bypassed, there’s no such thing as “backwards.” At least not near as I can figure.
In a couple days some fittings will show up that will let me connect the gauges anywhere in the system. Right now I’m can only connect them to the calipers. So this weekend I’ll connect them after the MC, after the bias valve, torch the damned nuts off the ABS pump, and check it there. This problem is not going to be able to run and hide.
It would be fascinating to find out that my “marketed as” OEM replacement MC that I bought from a local autoparts store has a small rear bore size that’s causing the higher rear pressure. It’s predecessor was exactly the same MC.
I’ve two different new MC’s inbound also. I’m on this like stink.
[attachment=2097]BrakePorpValve.jpg[/attachment]