Having a very hard time finding my brake problem

[jt1]

I've got the 15/16 ZX MC, a 74 booster, 1.75 Wilwood Superlites up front, 1.75 Dynalites in back, and a Wilwood prop valve. I used to have 1.375 superlites up front, and had a high firm pedal, but too much rear brake even with the prop valve at maximum reduction. I changed to the 1.75 supers, and now have a lot more brake, ie the car stops a lot quicker, but I now have a long pedal that's not as firm as it was with the smaller piston calipers.

 

This leads me to believe the 15/16 master isn't moving enough fluid for the 1.75 calipers to give a good firm pedal, although the overall braking performance is a lot better.

 

John

 

edit: needs a proofreader

[J__]

forrest i got pretty much the same problem as you..

i have 4x4caliper in front, 280zx caliper in rear, larger 280zx master cylinder, stock 280z booster, my brake pedal has a linear soft feel when depressed then suddenly bottoms. not like the normal "the more you depress, the harder it gets" feel, just soft, soft, soft, bam, hits the wall. and thats when i have the car off, when the car is on, the pedal is even softer.... i can pretty much depress the pedal with 3 fingers. i also bled the living daylights outta my brakes and checked for leaks. the only thing i adjusted was the push rod. i didnt measure how much i extended it, just by trial and error, when i extended it too much, i'd put on the MC and when i press the mc up to the booster, i can feel thesprings inside the MC "depress" a bit because the rod was too long and pushing on the cylinder. i then shortened the rod bit by bit until it didnt press on the cylinder, in other words "snug" with the 280zx mc. other than that, i didnt touch or adjust any other part of the brake booster/pedal system... i dont think the booster is broken. since it was working perfectly with the oem brake setup before i swapped in the 4x4 and rear disk breaks.... plz help

[Forrest]

I'm ready to set the car on fire.

 

I just replaced the booster, as my old one was nasty and had fluid in it, and bled again for 2 hours, 2 quarts of fluid. Another 14$ down the drain. No air at all in my lines, and the pedal is still soft. I've now replaced every part of the brake system with new parts except the original hardlines and stock porp valve.

[evildky]

I saw it mentioned already but didn't look like you responded, you did bleed both the inboard and outboard tanks of each caliper? also can help to give them a few wacks with a wrench and of course be sure the nipple is at the highest point

[Forrest]

Evil, yes I always bleed them both, outer, inner, then outer again. And I've tapped on them with a wrench every time I've bled them. I've lost count of how many times I've bled these things now. There's no worse feeling in the world than to spend 2 hours bleeding and bleeding and bleeding and see little or no air come out, then get back in the car and feel absolutely no difference.

 

I think my old booster was not working that well, and I wanted to eliminate that variable. With the new one in, the pedal is even softer.

 

Shoot me now.

[jt1]

How much vacumn are you making?

 

John

[JMortensen]

Go back and check every connection in the braking system.

[nismofiend]

Okay for starters I apologize if Im just bringing up a dead thread. I've searched everywhere for help and haven't gotten a straight answer but this thread seems as close to my problem as I have found yet. I've got an all stock '79 280zx 2+2 with the same issues as several of you. The brake feels excessively soft and spongy while the car is running but, firms up quickly when the car is off. The brakes work fairly good though although they won't lock up and the braking action is smooth as should be, I just have a terribly low pedal pressure.

 

I have new rotors and pads up front and the rear still has plenty left on it, I recently installed a new MC thinking it would solve the issue and I bench bled it until no bubbles came up, the whole system should now be air free and the fluid comes out clean, I have no leaks whatsoever but still the pedal feels this way, Im kind of scared to drive the car this way because Im not sure if it will fail on the street or if I will accidentally apply too much pressure causing an accident. Oh and I forgot to mention that the car has been this way since before I purchased it.

 

Some of you mentioned the reaction disc, to be honest Im not familiar with how the booster works but in my case could this be a possibility or should I attempt flushing all my fluid out and give bleeding another try?

 

Just as with some of you Im close to ready to kill the car, and I know for a fact that I have less invested in my whole car than some of you do in just your brakes alone so I know you've got to be pissed. lol

[J__]

ok just a little update, after endless house of bleeding my brakes still feel the same with that no resistance feel until it hits the wall and lock up. although its drivable it feels scary... reaction disk was in place and everything was fit correctly. so i took it down to my mechanic on fri because i had no clue left to what can be wrong. he took it apart and said that the rebuilt MC wasnt holding pressure(?) for the rear lines, possibly a siezed spring or something. in other words a bad rebuild. he's goin to take it apart and try to see if its fixable. otherwise i'll just get a new one. im hoping that is the fix to this problem... but that doesnt really help u forrest since u've changed 5 or so? mcs already? but maybe u were unlucky enough to get 5 bad ones?? or did u purchase new?

[5 Star Rising]

I noticed alot of you have similar problems as I do. Where your brake pedal goes to the floor,after endless Master Cyls,gallons of brake fluid from bleeding.Has anyone found and solved this problem yet lol?

Hmmm I wrench on trucks for a living and have never had this problem with brakes before. I have put 4 different master cyls on not including the one that came with my car. 3 remans and the latest one is a New MS. Still pedal goes to the floor,I bled the system 20 times with over 3 gallons of fluid ,even using a power bleeder. I replaced my front calipers with remans.replaced rubber brake lines and all short metal lines,bench bled all MS's,swapped my prop valve with another used one,checked and adjusted my rear drum brakes,made sure my caliper bleeders were on top,adjusted out my push rod that pushes my MS. When there car is off I can pump up the pedal firm and hold it, but when I start the car the pedal goes to the floor and the brake light comes on. I just bought the car recently and drove it for the first time tonight whoa it was scary lol I locked up my rear brakes on some fallen leafs on my street. Is there something I overlooked,any other tips would be awesome.. Iam ready to give this car back to the old owner for free! lol

[J__]

ok i posted my fix that solved my brake porblem already on the other thread. it was because the rear 240sx calipers werent meant for our cars so the bleed nipple isnt 100% at the top. therefore there was still lots of air trapped in it. i just unbolted the bottom bolt, tilted up the calipers so the nipple is at the top, put in a piece of wood between the pads so it can clamp onto something, then bled them again, a few good squirts of air later and the brakes are rock hard and engages right away. try it

[ezzzzzzz]

That's brilliant. I'll have to look at my calipers too. That could be the sole cause of reduced rear brake function in my 240Z.

[EMWHYR0HEN]

ok i posted my fix that solved my brake porblem already on the other thread. it was because the rear 240sx calipers werent meant for our cars so the bleed nipple isnt 100% at the top. therefore there was still lots of air trapped in it. i just unbolted the bottom bolt, tilted up the calipers so the nipple is at the top, put in a piece of wood between the pads so it can clamp onto something, then bled them again, a few good squirts of air later and the brakes are rock hard and engages right away. try it

 

 

Sorry to bring this up from the dead but there was alot of people who had this problem, including me, and it seems like this thread ended with no conclusion.

 

was this the solution to everyones soft brake pedal up top?

[EMWHYR0HEN]

Well I fixed my problem. The adjustable screw on the brake booster wasn't long enough even with it all the way out. I don't know if the depth of the hole on the 15/16 master cylinder varies, but the one on mine was deep.

 

What I did to fix it was cut about 2 cm of a grade 8.8 6 mm bolt and placed it inside the hole so that the adj. screw had direct contact with the MC.

[cygnusx1]

15/16ZX master new, AZC brakes all around, stock 280Z brake lines and all the gizmos that were stock on the factory brakes. I have a linelock and prop valve in the rear brake feed line (should make no difference to you).

 

The brake pedal feels firmer and reacts better and higher, tighter, than any other new car I have ever driven. It also modulates excellently. The brakes feel better than my friends new STI with the factory Brembos.

 

I bled the brakes the traditional method all by myself with a piece of hose that has a one way check valve in it. I also bench-bled the master before installing it. I use ATE Super Blue fluid and stainless braided, teflon lined brake lines at the calipers.

[cygnusx1]

Well I fixed my problem. The adjustable screw on the brake booster wasn't long enough even with it all the way out. I don't know if the depth of the hole on the 15/16 master cylinder varies, but the one on mine was deep.

 

What I did to fix it was cut about 2 cm of a grade 8.8 6 mm bolt and placed it inside the hole so that the adj. screw had direct contact with the MC.

 

Make darn sure that the "spacer" you made CANNOT fall out or you will lose most of your brakes.

[Forrest]

Funny that this thread has been revived. With the help of a friend of mine (who has just installed a hydraulic handbrake inline with his wilwood rears!) I installed the wilwood porportioning valve a week ago.

 

It made a huge difference. I don't understand why, but it absolutely did.

 

My pedal still feels like there is a lot of slack on top, but my friend said when he swapped in a new booster, it took a lot of adjustment in the piece that connects to the brake pedal to get rid of that slack. I would think that would only change pedal height and not slack, so I was surprised. But I will try that soon.

 

Once you get past that slack at the top, the pedal feels perfect. I have hope for the first time in over a year. Can't wait to get the last bit worked out, and then hopefully get one of those hand brakes for myself!

[EMWHYR0HEN]

Once you get past that slack at the top, the pedal feels perfect. I have hope for the first time in over a year. Can't wait to get the last bit worked out, and then hopefully get one of those hand brakes for myself!

 

 

That's exactly how it was for me before I fixed it. I could have just left it with the slack up top but it's kind of difficult to heel and toe downshift and brake quickly with modulation.

 

I have a good feeling that this is the problem with the slack on everyone's brake pedal. just to prove that, try putting something inside of the master cylinder and see if it shortens the slack.