Brakes get spongy when car is running

[branmr2]

I have a 95 240sx with a rb25 swap, the front calipers are 4 pistons from a silvia the rear are stock. I rebuilt one of the rear calipers and refilled with dot 4 synthetic fluid, then proceded to bleed until i got new fluid out of every caliper, master cylinder first of course. With the car not running the brakes feel good and stiff like they should but when i crank the car they get spongy feeling. I have re bled twice with no better results. I bought the car with the rb and brakes already installed and that just makes it that much harder to diagnose. Is there something concerning the vacume i should be checking, or the abs if i even have/had it? should it be a straight vacume line to the booster or should it have some type of valve in it?

[Kouki Chick]

If you push on the pedal and it continiously goes to the floor..sounds like a master cylindar problem to me...might want to check that out.

[onosqv]

the front calipers are 4 pistons from a silvia the rear are stock.

Spongy is a relative feeling. If you are running that setup w/ stock master cylinder, you will definitely get "spongy" feeling. Pumping the brakes w/ the car off should make it stiffer than normal, that's how it works.

Is it spongy, but still stops? If it hits the floor, you have a problem, if it is just soft but stops decent still, it's just because of your setup.

To "fix" it, if I'm correct. You need to at the very minimal change to z32 master cylinder - ideally z32 master cylinder + z rear brakes.

[ShionS14]

If the car actually stops when you press the brake pedal than you are good if it doesn't then you either have air still in the system or a problem at your master cylinder. Ultimately you will still want to swap to the 300zx master cylinder as it will give you better brake feel. Did you happen to know if the front and rear brake lines are a braided stainless steel line?

[branmr2]

No the lines arent braided stainless they appear to b stock. Im not really sure what all has been done i just assume the master cylinder is stock 240 but i could b wrong. The master cylinder and booster are spotless and appear to b new, so mabey the previous owner has already changed it? I dont think the previous owner really even knew what all had been done to the car because he left out several things on the mod list he gave me. But to awnser your question; yes the car stops, not as good as i would like but reasonably well. Im new to nissans and dont know all the abbreviations and numbers yet so help me out here; z32 is what car? what year? usdm or jdm?

[boznuttz]

The z32 and stock s14 bmc aren't that much different from one another to make such a significant change.

If you have abs, lol, it's been a pain in the *** for my brother, and it sounds like you might have to deal with the same problem. His abs module let an insane amount of air into the system making his life hell for a while. WOT pulls are fun and all, but not when you find the pedal dropping to the floor when getting off the thruway and that curvey off ramp. On several s13s that I've known of, abs has really caused more of a problem.

Search abs issues on NICO, JesusLikesKFC had to deal with this and might have made a few threads about it before. I'm not too sure what he was doing since I was on food patrol, but he may have just disconnected his wheel speed sensors.....I really have no clue on abs on 240s, so, SEARCH it up!

[ShionS14]

the z32 is the chassis code for the nissan 300zx if the brake lines look stock he may have done the conversion incorrectly by using the 240sx brake lines. the 240sx brake lines usa 10mm banjo fitting along with a brass washer to create the seal between the brake line and caliper, where as the z32 (which are also the same as the r32 nissan skyline of the early 1990's) caliper uses an inverted female flare to attach to the caliper. and all though the banjo can mate up to the z32 calipers it is still not designed to use that type of mating surface. as a side note the 300zx had 26mm calipers for non turbo and 30mm calipers for the turbo model, all r32 skylines had the 30mm caliper if i am not mistaking