I recently picked up a 1972 240z and I am preparing it to be my daily driving. With the aide of my friend, a great mechanic, we've completely replace whatever is possible to replace on the system. New caliplers, new drums, wheel cylinders, brake shoes and pads, master cylinder. However, due to lack of availibility, I just rebuilt the proportion valve (basically a hub where all the lines hook-up to with a pistion inside). Well, all is well, except we cannot build enough pressure to even stop the rotors, keeping in mind that there are no leaks in the lines. We thuroughly bled the brakes and we are certain that there is no air in the system. I'm thinking of taking back the master cylinder hoping that this will solve the problem but I dont know if that will do any good. In case that doesn't fix the problem I ask you if this is a good idea to maybe solve another issue:

Eliminate the proportion valve and just use couplers, isolating the front lines and the rear lines and let the master cylinder do its job.

Is this safe? The brakes should still work fine without the valve, right? I appreciate any advice you guys can give me. thanks :)

yes and no. you can eliminate the proportioning valve, but I'd put another one in its place just to adjsut the rear brakes. What you can do is take out hte stock prop valve, use a t-fitting off a 280z to split the front pressure to both front calipers, and use a wilwood proportioning valve to adjust pressure to the rears.

I would first check your MC/booster rod - it's adjustable and you probably need to extend it out a bit to get more pressure to build up.

Also, see about getting toyota 4x4 calipers to put on - all you have to do is bend a brake line, but you get 25-30% more braking force/area

Mmm...good call on the booster rod. I'll take a look at that , might be able to kill 2 birds with one stone. Though, before the rebuild, the front brakes worked, though the rear was completely disconnected. I'll check it anyways, thanks!

When I changed out my Master in my 280Z I had to replace it 3 times. The 3rd was a Factory new one. All the remans kept failing with in 6 weeks. just something to think about.

Yep, same here. I'm on my 3rd reman master cylinder in 6 months.