raptor99

Hello to all. I am newbe in need of some help with a brake problem. The car is a 1984 GSL with rear discs. The problem is that the brake pedal goes to the floor when pressing on the brakes. The pedal can be pumped 2 or 3 times and pressure comes back. I am on my forth master cylinder including the original and have the same problem. With the first replacement of the mc and total system bleeding the brakes worked fine for about a month. Replaced the mc again and the brakes kept the pedal pessure for two weeks. Now with the latest mc and bleeding, the brake pressure lasts about a week. When the system is first bled the brake pedal is stiff and has little travel. As the week goes on, the pedal goes to the floor. The master cylinder does not lose any brake fluid and I cant find a leak anywhere. What can this problem be? The brake booster? Proportioning valve? I am going nuts replacing the master cylinder and bleeding the system. Any help is greatly appreciated. Thanks

ZeikHunter

you check the brake lines? when i went to ss lines, i discovered that they all had cracks and bulges at particular spots. just because theres no leaks dosnt mean they cant lose pressure. even if its not the problem, you cant go wrong with ss lines

85 FB

+1 on the lines. Not uncommon that there'd be leaks found in them, I'd start there.

trochoid

iTrader: (6)

If the brake fluid resivoir, sp, is not going down, then the fluid is bypassing the seals in the MC. I find it hard to believe you have gotten 3 bad replacements. If there is any fluid loss, look under the dash, pull the carpet back under the pedals and look for fluid.

Otherwise you have a leak in the system somewhere. It's not the booster, when those fail, the pedal will get rock hard when pushing.

Now, what I need to ask is, are these new or used units you are using for replacements, if used, get a new one. If you are rebuilding them yourself, get a new one. Are you flushing the entire brake system when bleeding? What condition are the soft lines in?

Btw, Welcome to the forum and the Darkside. In my sig line is a link to the online FSMs, carb manual and other reading material.

aussiesmg

iTrader: (1)

Have you changed the slave cylinder when you did the Master as they wear at the same rate, I think they should always be changed together.

85rotarypower

Uhhh, last I checked there is no slave cylinder in the brake system.

It definately sounds like a bad master cylinder. If you fluid level is not going down, then its got to be the master leaking internally. Any other problem would have the fluid level going down over time and the pedal would not come back to life after pumping a few times. Every time I've seen a problem like this its the master cylinder.

Rx-7Doctor

iTrader: (13)

It's very common to have exterior cracks in rubber flex hoses. And the only way for a hydraulic system to lose pressure is to lose fluid. The most common failure for brake hoses is the breaking of the internal structure causing them to collaspe. At this point what you usually get from this is the brakes pulling to one side. Meaning there is no longer good hydraulic pressure with the collasped hose so the brakes on that side are useless.

raptor99

Thanks to all for the replys. The first master cylinder was what was called a "brand new" unit off of ebay. This worked for a couple of months. The other two units were rebuilt models from the local Car Quest parts store. The second one being a warranty replacement. The brake lines are the original also but there is no fluid loss and the car does not pull when on the brakes. It seems that the master cylinder is what most think is the problem. I didn't think so because I am on my forth unit and is it possible to get that many bad units? Does any body have line on where to get a new one at a reasonable price? The dealer wants almost $300.00 and I am afraid to spend that kind of money and not have that be the problem. Thanks again to all and any more suggestions are greatly appreciated.

Rx-7Doctor

iTrader: (13)

It is possible that you are not getting out all of the air in the system. How are you going about bleeding the system?

illbill

I noticed on my 85 gsl-se, in which i am stripping the interior, that my master cyl. was leaking in the cabin. Much rust removal now. definitely pull the carpet and look if you are leaking there.

trochoid

iTrader: (6)

1st rule thumb for rotarys, do not go to the dealer for work, period. If you do, bring vasoline, with sand. If you can, buy the Tokico master cylinder. Instead of going through the local auto parts store, try BlackDragonAuto.com or Rockauto.com.

Are you bench bleeding the master before install and are you getting clean/fresh fluid from each of the bleeder valves? During the bleeding process, do not let the fluid level drop too far in the reserviour. Bleed order is right rear, left rear,, right front and lastly, left front.

When the problem reoccurs, after pumping up the pedal to get a firm pedal, push and hold it. Does the pedal still go down slowly? If the pedal holds, you still have air in the system, if it slowly goes down, you have a leak in the system.

sgieldon

No one mentioned checking the Vacum Line going into the MC.
Don't forget there is a Check Valve in there.
One of my past RX's had that thing work sometimes and not others.
I went nuts trying to figure out my Brake Pressure Problem to include replacing the MC and all the Lines to SS.
Someone on this Forum suggested to check that Valve and sure enough that was the culprit.
Not saying thats your problem but It looks like that might be one more thing to check.
If you decide to replace your Brake Lines (Go Stainless For Sure).
Good Luck and Welcome.
sgieldon
steve

Rx-7Doctor

iTrader: (13)

Steve, As Trochoid had pointed out earlier in this thread. The booster has nothing to do with the pedal travel. If their is a problem with the check valve or booster you lose the power assist portion of the brakes and the brake pedal becomes hard.

sgieldon

Hey Doc;
Ya I got that part.
But I had this simular problem that Raptor99 is going threw and after alot of money later found out that if that Valve is geting stuck in a open position ie won't hold the Vacuum it can cause the same problem. Just how mine turned out!
I'm just puttin in my .02 in to check since everything else has been covered.
sgieldon
steve

raptor99

Wow! Thanks guys. But I am still confused. I have checked for some of the things that have been pointed out. There is no fluid leaking out on the inside firewall of the car. Behind the carpet is clean and dry. I pumped the pedal three times and the the pressure holds and does not go down. I guess it must be the master cylinder. I have been using rebuilt units from Car Quest and a " brand new" unit from ebay. I will try to find a new unit. When bleeding the system I start by bench bleeding the master cylinder. The only strange thing that has occured on all the units I have used is that when bench bled, fluid comes out of the front and bottom hole of the master cylinder but not the rear most hole. I must have pumped the master cylinder a thousand times and no fluid out of the rear hole. I then bleed the right rear, left rear, right front, and left front. I have noticed that the pedal pressure does not come up till the right front and there is always alot of air bubbles on that wheel. When the whole system is bleed the pedal has very little play and very little pressure is needed engage the brakes. This lasts a short time though. If I re-bleed the system I can get the pressure back. I must have gone through gallons of brake fluid and all the fluid in the system is clean. Any feedback on the procedure I have used for bleeding? Is it normal for fluid not to come out of the rear hole in the master cylinder when bench bleeding?

raptor99

Anybody? Please?