I've been reading about swapping in rear ends with discs and the need to change to a disc/disc prop valve but haven't seen anything about prop valves particular to V6 cars. Am I right in thinking the weight difference/distribution between V8 and V6 cars would require a different prop valve? If I put a '87 9 bolt in my '90 V6 what prop valve do I need? I also plan to do some lightening and weight redistribution (a/c delete, fiberglass hood, battery reloc. etc). Is there a fixed prop valve to cope or would it be better to just go to an adjustable set up to deal with the ongoing changes and tune for auto-x? Any adjustable valve advice?

 

with all the variables, it may be better to just install a universal adjustable valve and tune it from there.

 

So I've done some more reading on this and it looks like the only (non electronic) way to get true linear front/rear adjustable brake bias is with at least dual masters with appropriate cylinder diameters and an adjustable bias bar. Adjustable proportioning valves are not truly linear and only adjust the rear by reducing pressure. So if you already don't have enough rear pressure the adjustable valve won't help. I'm assuming that a fixed disc/drum prop valve supplies more pressure to the rear than a disc/disc valve. If that's the case then I'm thinking that if I ever get to replacing my drums with discs I can leave and the disc/drum fixed valve in place a put an adjustable valve in the rear line to reduce the rear pressure so the rears lock up a little after the fronts. Maybe the the fixed prop valve can or should be removed completely. I'm not sure how the rear would behave with both in place. Has anyone done this? Has anyone installed dual masters? Any other thoughts or suggestions?

 

my suggestion to install the adjustable valve requires the removal of the factory prop valve. so the only limiting factory on pressure if the master cylinder itself.
something like this?


the front lines are t'ed together and run straight off the master, and the rear is adjustable with the valve.

i may be wrong but i believe the disc propvalves provide more pressure than the drum ones, but in most cases, still not enough to function properly. unless you use a later valve for the 1le cars or rear pbr calipers, not the iron ones. they are different. I think they changed in 89? you would need a later valve anyway since the threads for the front line on you 91 is different than the early pre88? valves.

 

I found this adjustable valve from Summit racing. SUM-G3910. Claims to be universal and has 3/8" -24IF fittings. Has anyone fitted one (or similar)? Do our brake lines have this size fittings already?

 

I should have typed faster.

 

The Summit valve has two front outlets so separate lines can be maintained. I thought the disc may need less pressure than a drum for equivalent effect. But I don't know. Anyway this looks like the way to go.

 

yes you should young grasshopper.

no, our cars have bubble flares, you will have to cut and flare you lines to install the prop valve. there are ways to gut the factory valve, but that doesnt look as clean.

all these aftermarket valves have a 1/8-27 NPT thread in them, then they give you the adapters for the double flares. I ditched them and just used AN flares for everything since they are so easy to make and seal wonderfully.

 

I wish the operative word there was "young"!