Baffling S14 brake issue

[Scott Buwalda]

Help me guys, this is a new one for me.

Noticed recently the brakes on my red S14 were mushy, so I decided to finally get around to putting my 26mm Z32 brakes on it today. Put the passenger side on first, and took it for a ride...nice braking improvement. Was driving along, and noticed it getting mushy again after a few minutes of HARD testing on the pedal, and by the time I got home, you could put the pedal to the floor before the brakes engaged. Noticed oil leaking from the reservoir cup (around the seals)...removed the reservoir and pushed it back in, cleaned the area off, and worked the pedal a bit, and no more leaks.

I figured it was an air bubble in the master cylinder, so I purged that valve too---nothing but oil.

Went on now to the driver's side with the Z32 brake swap. Both sides were thoroughly purged with a hand-held vacuum pump...extensive purging until nothing but oil was returned from the purge valve. Buttoned her all back up, and before starting, pounded the brakes, and within four or five pumps, you couldn't move the pedal any further; rock solid. Start the car, and the pedal drops to the floor. The brakes do work, just not good, and there's a lot of pedal action before they even begin to engage.

I am thinking master cylinder. But why would this happen only when the car is running? I also noticed that the reservoir cap rubber gasket is bad, and I removed it...perhaps this is a problem? Help anyone? Need to get this thing on the road tomorrow.

[180fan]

probably a master if you are indeed using brake fluid like I'm gonna assume you are. Reason it'll hold is because your brake booster is working with no leaks.

[Bronze MFP]

check your rear brakes if you havnt already, i didnt see any mention of them in the post. also there is a distribution block type thing near the trans tunnel on the passenger side that connects 2 hardlines that go to the rear brakes. check that as well as mine was leaking from there and had me scratching my head for weeks as to why i was losing fluid and pedal pressure but there was no fluid leaking from any of my calipers/hoses

[contranication]

Well, if you put on z32 calipers which is what I think you ment by brakes, then you should have bled your entire system from farthest wheel from the master cylinder continuing to the drivers side rear then passenger front, then drivers front and finally the master cylinder itself... not just the front, that is to fully ensure that there is absolutely no air in your system. Before you do this you should definetely replace your reservoir cap with one with a gasket so that you can further make sure that no air is seeping into your system(rare but it happens)... make sure when you bleed your brakes you completely shut the bleed valves and check that your brake lines all over are not leaking anywhere.

Also pumping your brake pedal right before going for a test drive doesn't do anything b/c all you are doing is compressing air if there is any in your system, the pedal should be hard when pressed at around the drivers side wheel.

Hope this helps

[ISUJinX]

Switching to Z32 brakes, the pedal will feel mushy. After all you are putting extra pistons into the system. I've heard a lot of people switch to the bigger master cylinder and their problems dissapear.

[contranication]

Mine never felt, mushy not one bit, with a stock MC but I did switch to racing brake fluid

[drifterXtreme]

Some people wouldn't call the feel of z32 brakes mushy on the stock master cylinder, but there is a huge difference when you start using the 300zx master cylinder. It will make pedal modulation a lot easier. the stock MC tends to have a bit of a hesitation and then the fronts have a tendancy to lock easily as they finally get enough force to clamp... makes for a good way to flat spot some tires. I've found it all to be a matter of taste what one person thinks is mushy another says is fine untill they actually get to feel what its supposed to be like. As far as the problem goes you really need to bleed everything before making any conclusions...