Just recently I have noticed that once in a while (not every time) the when I push the brake pedal that is goes down about twice as far as it usually does before I feel pressure. There is still plenty there to stop me when it does this. Like I said it only does this once in a while for example on say a 30 min drive it will do it 3-4 times. When it does it, if I let off the pedal and then push it again it is fine even if I wait for a few seconds so it isn't like I am pumping them.

The brake fluid reservoir is full and not losing any fluid. The rear brake calipers were rebuilt a couple months ago along with new brake pads (front and rear) and there is no sign of fluid leakage on the ground any where or any of the lines or calipers.

Like I said above it only happens once in a while. I use the brakes from one stop light to another 3 or 4 times then it does this then at the next light it might do it but not always.

If anyone has any ideas what to check I would greatly appreciate the help.

 

I was about to post my problem on this but you have described exactly what I'm experiencing on my '85 GSL. I got new brake pads very recently as well, couple of weeks ago. The only thing I can think of is bleeding the system to remove any air on the hidraulic lines. But I'd also appreciate any additional advice on this.

 

erick...
What youre describing is not brake fade
Bleed the entire braking system for traces of air and if the problem is still occurring then its possible that your brake master cylinder is beginning to fail ... time for a rebuild or better yet get a brand new OEM one.

 

It's possible that your brake booster may be failing, but usually the car stalls at stoplights due to vacuum loss.

 

Brake Master Cyl Is What you need. If you didnt bleed the syst. when you did your Previous brake work There is a possabilty of air in the lines but it wouldnt be intermiten

 

Exactly. Air wouldnt be an intermittent problem %99 of the time. Sounds like the master is bypassing.

 

Speaking of brake boosters, I've noticed that I can nearly stall my car at idle (-300 RPM) by mashing the brake pedal repeatedly.

I'm not sure if this is normal or not, as I don't have another FB to compare mine with.

 

mine doesn't do that, so yours must be abnormal.

And how do you get yours to idle at -300 rpm?

Sounds like a vacuum leak to me, somewhere between the booster and the intake manifold. You can clamp the booster hose at various points between the booster and the manifold and see where the leak is (vice grip works, don't forget to remove it before you drive). See if the idle still drops when you mash the brake pedal. The leak may be within the booster itself. There's a little tutorial on brake boosters at auto.howstuffworks.com

 

No, I mean, it just looses about 300 rpm when you press the brake pedal fast enough. If you just hit the pedal once, it doesn't do it.

My car is an automatic if that's significant.

 

Inside the brake proportioning valve (attached to the front of the booster, on which your fluid reservoir is located) is a plunger that effectively captures and pressurizes brake fluid in the lines leading to your calipers and drums.

This little plunger has several different passages in it to allow for fluid to come from the reservoir and return to the reservoir based on whether you're pushing down or letting up on the brake pedal. There are thin plastic valves that open and close to allow more fluid to be pulled in, such as when the brakes wear and require more fluid in the lines to fill the void created by the pistons being out farther.

When this plunger gets plugged up, it can cause fluid to recirculate back and forth between the front and the back of the plunger, causing erratic brake pedal feel as you described. This is due to the pressure sometimes being lost as the fluid passes through the 'ports' and past the thin plastic valves, which effectively bleeds off brake pedal pressure.

oftentimes, 'pumping' the brake pedal can free up the crap stuck in the port, or even free up a stuck plastic valve, which will allow the brake pedal to return to normal for a period of time.

The fix is to either replace the brake master cylinder proportioning valve as a unit, or to rebuild the brake master cylinder with one of the commonly available rebuild kits - this replaces the plunger, the plastic valves, and usually doesn't last as long as replacment as a unit. HTH,

 

Thank you for all your input. I will double check the vacume line to the brake booster and will probably look into getting a new mastercylinder and proportioning valve. The brake system was completly bled after the work on the calipers was done and was fine for a few months up til now. Thank you again.

 

Well it was the brake master cylinder for me. It's replaced now and the brakes are back in excellent shape.

 

AHHHA!!!!!!!!! How did you bleed the system? If you did it by having someone open the bleeder, and you pressed the pedal all the way to the floor to push out the fluid, thats you culprit right there.

 

This is the number one way that most of the world bleeds their brakes isn't it? What's wrong with this method?

Bran

 

Your supposed to have someone pump the pedal a few times(and hold) to build some pressure THEN open the valve

 

you mispelled your name Brian.

Anyway, negatory there good buddy, pull up a chair and lemme tell ya a story about brake bleeding. So were all on teh same page lets look at a master cylinder (MC).



Now in normal usage, the piston only moves part of its possible full travel. Teh rubber seals move back and forth over a small portion of the cylinder bore. Well a funny (actually an irritating) property of brake fluid is that its hygroscopic. Hygro.........wtf? Basically it has a propensity to absorb water. It absorbs water from the atmosphere because the cap is vented. Over time a considerable amount of water ends up in the fluid. Water being heavier then the brake fluid, it like to settle in the lowest point it can. If left long enough, it will rust the brake lines and components from the inside out. Thats why its reccomended that you flush brake systems every couple of years minimum. Anyway, I'm starting to digress. So we have this water in teh MC. It settles at the bottom of the cylinder. Where the piston seals rub back and forth is no problem (its basically constantly getting polished and stays shiny and clean). But the part of teh bore that doesnt see any action starts to rust. This creats a rough surface. When you go to bleed it and puch the pedal to teh floor, you cause the delicate rubber seals to pass over this rough, sandpaper like surface. This damages the seals. And then for a while your fat dumb and happy, and all is well. But the seals are damaged, and now are starting to get chewed up with every push of teh pedal. Given a few months, your MC starts bypassing. And now you get all pissed cause your brakes are going bad again.

Moral of teh story: The "to teh floor method" is only ok for a new MC that has a clean bore. For a non new MC, push down no further than you would normally move the pedal when bleeding. OR use/make a pressure bleeder. I made one from a garden/pump sprayer. Best way to do it.

 

I replaced the brake booster but no change. I am replacing the master cylinder tomorrow. I figure that will fix it. If that doesn't work then I will look into the propotioning valve.

 

Can you explain how you did this?

 

Same thing happened on my GSL-SE. I bled the system but it didn't fix the problem. Then I noticed the big puddle of brake fluid behind my driver front wheel.

New MC was about 135 shipped from Mazdatrix. Piece of cake install if you have a Haynes, and everything was gravy after. Bled the system and had a very nice day.

Gluck

 

I changed the master MC yesterday. That fixed the problem. I also took apart the proportioning vavle and cleaned it out. The brakes feel like brand new now. Thank you for all your input.

 

Yeah, that's what happens when I type too fast and don't double check before I submit.

Oh, and thanks for the tutorial on brake bleeding.

Brian <-- Got it right this time carl!!!