Tron

If I am sitting at a light with pressure on the pedal, the pedal will lose pressure and eventually (after like a minute or two) I can push it to the floor...I don't seem to be losing any brake fluid, cuz the master has not lost any fluid level and there isn't any leakage anywhere...

I took it to Midas for a free inspection and they said the master cylinder was fine.

What process should I use to go about finding the problem? I was thinking it'd be a good idea to replace the calipers and pads all the way around because they are original with 175k on them, but if there is a cheaper way I'd prefer that...

Any suggestions?

Thanks in advance,

Brett

blu_gxl

sounds like either your check valve in the vacuum booster or the booster itself

oh and i wouldn't trust midas to tell me what kind of car it is they are a bunch of inept fools

Tron

More info - the car doesn't pull to one side during hard braking either so I am thinking the calipers are still good...

opelbits

iTrader: (5)

I have the same problem. No air in system and it doesn't get worse. The pedal just likes to 'sink'.

DaleClark

iTrader: (55)

Bad master cylinder. The internal seals of the master cylinder are leaking and bypassing pressure.

Replace it and you'll be good to go.

Dale

Tron

This pisses me off badly because before I parked the car for a couple years (yes I started it every two weeks and ran it around the block) I had the m/c replaced at Midas. I took it back like 10 times telling them it didn't feel right.

Bastards! I hope I can figure it out...

Turbonut

iTrader: (1)

There's got to be a reason the pedal sinks. While parked, push pedal numerous times, then check each wheel. I don't mean just look from outside, I mean remove brakes and check pistons for seepage. There doesn't need to be much leakage for the pedal to sink. If none found make sure master cyl not leaking by the booster. If none found internal leak in master cylder...Fluid leaking around piston, or booster.
With engine off, pump pedal numerous times to relieve air from booster, then try pushing hard on pedal to see what happens.

Tron

I put a new master on, still the pedal sinks slowly and i have horrible fade when repeatedly applying the brakes from freeway speeds...

No leaks anywhere that I can see.

Turbonut with the car off and pumping the pedal numerous times and then pushing on the pedal it stays firm and doesn't sink like when the car is running.

Thank you for the help again I appreciate it.

Brett

Tron

If my caliper pins were not lubed very well would that cause the sinking pedal?

Juan

iTrader: (28)

Are your brake lines any good?

Is it possible for them to expand and make the pedal feel softer? I dont know if they can actually expand that much but its just a thought :p

Tron

the brake lines are original, it's possible for them to expand but it sure seems like the system is losing pressure somewhere with the engine running. as i said, with the engine off, the pedal stays hard and doesn't sink after a few bumps to relieve the booster of air...

marlaman

Yeah i was thinking alone the lines as Wan. My dad has done a lot of brake work on quite a few cars in our neighborhood and I asked him if it could be expanding brake lines. he said it is most possible for that to happen. not very likely all weak tubing but yes it's possible.

Have you ever used the brakes on a car with stainless steel brake lines? From my expirence those are the best if you get them done right. it's a very tight and crisp feel to the brake pedal.

thats just my .02 for what it's worth

Silkworm

ABS? ABS unit may be leaking or dead.

Tron

It doesn't have ABS...

Has anyone ever replaced the check valve or the booster? I am going to order some SS brake lines I think too just for an upgrade anyway - that is long overdue...

opelbits

iTrader: (5)

I've replaced the master with a rebuilt unit and it felt the same. Immediately replaced that with a brand new one (all the work was done by a shop). Still the same.
When your pedal is sinking, do you hear clicking sounds from the calipers (do you have 4lug brakes or 5 lug?).
Does anyone know if the master cylinder is different for 4 lug versus 5lug? It has been suggested that the 4 lug master may not pump the same amount of fluid in the front circuit. I have reason to doubt this idea, but you never know.

SilverT2

Had the same problems on my non abs vert, I replaced the master cylnder and brake booster, and it soved the pedal sinking. I now have the same problem with my T2; the ABS kicks in and the pedal sinks.

tesla042

yeah..exact same thing happens to me... sinking and fade... but no sinking when the car is off... I'm assuming that means it's the booster, as that would be the only thing that's different with the car on or off..

I have put 3 SS brake lines on, but with this problem, i can't really notice a difference.. The compression fitting on the LR line is all stripped to hell, so i haven't been able to get it off. Maybe some day..heh

I think the 4-lug and 5-lug MC's are different... I have a semi-parts car laying around that's a 4-lug, and I was thinking about swapping them, but someone recommended against it.. If I wasn't feeling so lazy, I'd go check the part numbers on mazdatrix.com.

-Tesla

SureShot

This is not that complicated. If the pedal is sinking - fluid is going somewhere. If the reservoir level is going down, it’s an external leak. If the reservoir stays full, it’s an internal leak in the master cylinder.

opelbits

iTrader: (5)

Logic would tell you that. But if that is true, then the original mc, the rebuilt unit, AND the brand new one were all bad in the same way. Possible, but not very likely. I am more inclined to question the abilities of the mechanic that did the work. But, then again, I have no fluid leakage. One thing I've learned in the years I've had cars is that if there is an unlikely, almost unbelieveable way for something to have a problem, I will be the one to experience it.

Matlock

Nope, not the only one (points to self). I am right with you.

One other thing to try would be to rebuild your calipers, a little pricey for all four but, it's a suggestion. Even if thats not it, hey you have rebuilt calipers and you know they aren't causeing it. Good luck!

Tron

What I really need to do is take it to the dealer and have them pressure test the Power Brake Unit (booster) and also check the Proportioning Bypass Valve.

My calipers should not be bad, but you never know...I have wanted to get new calipers anyway...

SureShot

Yeah, we all run into bizarre problems, but I'm the optimist that wants to start with the simple or cheapest thing first.

blu_gxl

i still say it sounds like you booster i'd go have them test it for you.

also there is a difference in the master cylinders the 5 lug brakes usualy have the 4 piston calipers so the master should have a slightly larger piston with those caliper.

opelbits

iTrader: (5)

But if the mc piston is different, what about the rear brakes? Wouldn't you at least get a firm pedal from them? If the rear calipers are the same, you should get a hard pedal that doesn't sink. Is the proportioning valve different?
As for wierd things happening, I am apparently the only person I know, or any of the mechanics/shops I deal with knows, that has ever had 'carbon lock' in a piston engine that runs on gas.

Nick86

iTrader: (1)

Who says that it's all just one problem?
The reason it doesn't fade when you push on the pedal hard and quick is because the fluid can't displace it's self around the seals in the MC quickly. If you press slowly, it will push the fluid around the seals. If you can only get this to happen when the car is off, then the booster is the problem. If it happens all the time, the MC is the problem. Or, it's a combo of both. If it's not leaking fluid, the MC is the only real place where you can get that magnitude of a problem.

I can't remember, have you bled the whole system? If not, THAT is your problem. Especially when a car has been sitting for so long. If you havn't already, bleed all the brakes until the whole system is completely flushed of the old fluid. You'd be surprised just how ineffective water and dirt are for brakes!