Mushy Brake Pedal, already searched forum
 

The last couple of days I have been loosing brake pressure on my 89 GTU. When I hit the brake pedal it slowly will travel all the way to the floorboard and eventually the braking effect goes away and cannot stop car without application of the emergency brake.

I suspected loss of brake fluid but reservoir is full and can't find a leak. Next suspected bad Master Cylinder but can't see any leaking around it on the firewall or connections to the MC. My only other concern is air in the brake lines. From the forum search I might also have weak brake lines or the brake fluid might be worn out. I don't think it is a rotor or caliper operating problem; but if 1 caliper failed I should still be able to stop the car with the hydraulic system, just might take a bit longer, right?

First below freezing night happened just before this started yesterday morning. Could this temp change have any damaging affect on any of the brake system components?

Any ideas to further diagnose the problem or suggestions on fixing it?

I guess master cylinder needs rebuilt or replaced...When's the last time the fluid was changed?

MC, as far as I know its original to car,

Brake Fluid, not in the 4 years I've owned the car. It is pretty dirty. Would DOT 3 fluid loose its properties overnight or suddenly?

No, but 4+ year-old fluid would have some water in it from the moisture accumulation over the years, and if the water freezes into little crystals...

I rebuilt mine during the engine rebuild, took me about 45 minutes, and replaced all of the fluid while I was at at. Got the kit from Mazdatrix, I think, although I'm sure you could get non-OEM kits from the auto parts store...

If your pedal goes to the floor and you are not losing fluid then it has to be the master cylinder is bad.

It's always a good idea to change the fluid at least every other year or any time the fluid is not clear. The fluid does not really go bad but it does absorb moisture and that can allow the system to degrade. Moisture can also allow the fluid to boil at a lower temperature but you would only notice that in heavy repeated brake use.

I have never been able to bring back a brake system with anything less than a rebuild. Changing the fluid now won't hurt but the damage has probably already been done.

ed

A pedal going to the floor is definitely the master cylinder. You could also look into SS brake lines if you're still not satisfied with the feel, those will help with the pedal as well.

While you're doing all that, if you like the feel of having a tight brake pedal I would really recommend stainless steel brake lines. They made such a great difference on my Subaru.

What they said. If the fluid isn't leaking anywhere, but the brakes don't work, there's an internal leak in the master cylinder. If the brake fluid hasn't been changed lately, there's likely (a lot of) water in the fluid, which could cause problems.

Rebuild time! Just not the engine (for once).

-=Russ=-

I have been wanting to upgrade the system with the SS lines and since I have to rebuild the master cylinder and change out the fluid and bleed the lines now seems the best time to do it all.

I appreciate the diagnostic help and will do it right.

Bert