Removing bias valves from 300zx master cylinder, general bias?'s MC size?'s

[kclo4]

Hullo brake experts.

I am wondering if it is possible to remove the rear bias valves from a 300ZX or 240SX master cylinder to leave it at a 50/50 pressure split. Then I can use a proportioning valve on the rear if the rear has too much force.

I have 26mm Z32 brakes up front and the stock MC and there is just too damn much bias up there for my tastes. I am going to upgrade to a Z32 MC but I haven't decided 1" or 1 1/16". I don't have ABS.

I know the miata guys have it easy, their stock proportioning valve is external to the MC, but we don't have that tuning luxury.

Looking for feedback on the MC sizing issue. I have a manual 240, and don't want to swap brake boosters. I would like to improve brake modulation so I don't want it too stiff, but I don't want it mush like it is currently. Any opinions on MC size?

In the event that removing the stock proportioning system is not possible, is it reasonable to put a proportioning valve on the front brakes? Do they make proportioning valves with 2 inputs and 1 output? Is it reasonable to reduce front braking force in general? How does that effect tuning the pedal feel?

I have searched and found some vague answers to some of the questions, nothing for the rest, so hopefully I can get some insight as to my upgrade path.

Thanks

[RobertsnewQ]

Well, you wouldn't want to put a prop valve on the front brakes. Here's why:

Prop valves (at least the aftermarket ones) don't really vary pressure as a function of input pressure at all pressures. They have a "knee" point where the curve changes and reduces pressure as a function of input pressure.

Sooooo if you put a prop valve on the front, at the knee point it will reduce front brake pressure. The rear brakes will have a different knee (factory prop valve) and so your proportioning will vary with brake pressure. So you'll have different brake bias depending on how hard you press the brakes... Not the best situation.

Plus you will never get maximum brake pressure to the front brakes - the prop valve will reduce front brake pressure. For most effective brakes you want full pressure to the front and varied pressure to the rear. This is because the harder you stop (before lockup anyway) the more weight transfers to the front axle and the more front brake pressure you want. Reducing front brake pressure is the OPPOSITE and will increase pedal travel and force at the wrong time (just when you want the brakes to work harder).

Hope this makes sense.

I would go with the Z 1.0625 mc, since it will give you less pressure (a firmer pedal). THEN experiment with bias. I suspect that you cannot remove the bias valve from the factory MC but I could be wrong.

Also, the Z MC may have different bias than the stock MC, so you should try it before committing to anything.
Modified by RobertsnewQ at 11:07 AM 7/23/2005