A Brake Problem/ Question

[Matt Demick]

Ok this is for my GF's 240sx but brakes on cars are almost all the same

So its a non abs car first off.

If u are at a light holding the brake for a few seconds u can feel the pedal go farther and farther down slowly. she will begin to roll if on a hill but if she lets off and applies again its ok for a few seconds again.

Now in the past 6 months i've done front and rear rotor, pads, and a master cylinder. I've inspected all brake lines and connections are there are no leaks. I've also bled the system muiltiple times including 10 times each corner yesterday.

I'm thinking its the brake booster. but a new one is over $250 so its a expense guess. What do you guys think?

[Merlin Beerens]

I agree, it sounds like a booster.

[Dan Harshey]

can you hear any sort of vaccum leak type sound when the brakes are on, compared to when there off?

[Matt Demick]

i never listened for a leak. will tomorrow when i get a chance

[Derek Fung]

when reapplying the brakes, the brakes work, so that eliminates any fluid leaks in the system.

see if there are any cracks in the vacuum hose going towards the brake booster.  if it checks out fine, most likely the booster is the cause, as you have replaced all the other components.

[mitchell hamilton]

Sure it's not the master cylinder? ( I know you said you changed it though) As you hold the brake, fluid goes past the internal seals, pedal goes to the floor. Sounds definately like a internal fluid pressure problem to me.

[Matt Demick]

good point Mitch. Well its under warriente from Lordco. got it like 3 months ago. So i'll bring one in just incase. But a buddy has a Booster in his garage and hes gonna give that to me for free. so i'll slap her in and see

[KevinBuckham]

Was this an OEM master, or a knock-off brand?  The knock-offs are pretty notorious for being bad right out of the box.  I've had friends go through three or four, finally give up and buy one from the dealer that works perfectly.

If you are loosing vacuum, the brake pedal will feel hard and it will be difficult to stop.  The brake pedal dropping usually points to an issue with the master, fluid weeping from the connectors, or even leaking past a caliper piston seal.

Sometimes caliper seal leaks can be hard to see as the fluid gets spread around everywhere and the inside of the wheel just looks a bit wet and dirty.    I've even had line leaks that were hard to spot as the surrounding area just looked very slightly wet and dirty.

[Matt Demick]

i had a aftermarket one in there and it lasted a day then i put a OEM one in. Ordered another OEM one as a just incase. pedal seems to be sinking when the car is on and off now. so deff sure its the master. BUT i will take a look at the rear calipers since i did pads and rotors on the last month

[John Hartman]

I got in here a tad late, but you seem to have it in hand.  I would really suggest a master. Bypassing fluid like Mitch said.

And for knowledge, when a brake or clutch system has air in it, it WILL get better with repeated pumping up.  EG, in racing, you will often see the driver pumping up the brakes and then they complain about boiling fluid.  See when it boils, it creates air(gas) pockets.  Pumping it up temporarily takes care of that.

Also, the master has to be bench bled before its installed.  Look it up so you see how.

If not, you can get a bubble of air stuck in the perportioning valve.  To get rid of this, have 2 people.  person in the car, person with wrench to crack bleeder screw.  First, open screw, then push firmly all the way down on pedal, not too fast, but smoothly and quickly.  Hold it down, close bleeder.  You should hear a distinct "click" when the damn thing resets.  Do farthest away wheels in order to closest.

Also, if you must test the booster, pretty simple, just unplug the vac connection and plug it off so the engine doesn't run shitty.

Your symptoms sound hydraulic to me for sure. Could be a caliper or perp valve or other connection or leaking hose/pipe.

Really clean up each caliper, the whole thing, so if any leaks are evident they will show up.  Clean em and dry them.  Drive car for a day or so, make sure the master is topped up.  Put it up in the air(one wheel at a time if you have to).  Use a strong light and really look for leaks.  This can also be done at any connection and at the perp valve.

I had this on Carmens TSi and it eluded me for several months of on-and-off repairs.  Once the leak in the caliper got a bit more severe(but totally still driveable, most people wouldn't even have had a problem with it) I found the leak.

[Matt Demick]

Ok so i went over every brake line, porportioning vlave, caliper, no leaks anywhere.  and now just changed the booster. Still the same.  ABSOLUTELY no leaks anywhere. Resivor stays at the full line.

So i've come to the conclusion that the piston seal is screwed inside of the master cyl and fluid is getting around it. My new one will be in tomorrow morning so i'll change it tomorrow after work.

[Matt Demick]

Got the new master in and she works fine now. thats the 3rd one this year. i got the most expensive OEM one Lordco carried and then they ate the bill.

Thanx for all your input guys