So i've replaced my mastercylinder 2 times in 4 months. Now i've got a soft brake again.. i'm gonna bleed the brakes before i try anything else but it seems as though i've got another leak. any ideas? new brake lines needed? check the brake lines? etc.? Thanx.

 

Check the obvious are your calipers leaking?

 

Are you sure your booster is still good? When bleeding brakes on my '79 Mercedes 450 SL, I pressed the pedal a bit too far and ripped the diaphram in the booster. When the pedal was depressed, I could hear a very loud vacuum leak coming from the firewall area. Rather unusual that the booster goes, but it is a possiblity.

Did you take apart the two bad MCs?? Were the seals torn? I would start here (you only need to remove one c-clip and a small screw from the bottom of the MC to get to the seals....depending on year). If the seals are damaged, rebuild kits are available for only a few bucks. That is the route I went when I lost the brakes on my '68. New seals, and a nice hard pedal.

Good luck.

~Rich

 

Were they rebuilt or new master cylinders? You probaly can guess where I'm going with this....

 

:iagreee:
Never buy rebuilt Master clyinders, it's not worth it.

If it's a new one, caliper seals, rotor runout...

One way to check the MC is to push on the pedal, if it's hard but slowly sinks to the floor it's bad.

If it drops to the floor on the firts press, but pumps up, does it stay up? if not it's bad, if it does you prolly have air in the lines.

 

From your posts, I understand you have a '71 and there should have already been some work done on your brake system. There are about a gazillion things that can cause a soft pedal so it's really hard to diagnose from your description. Since there is about a 20% chance of getting a bad part out of the box your chances of having two sucessive failures are low (although I have had that happen on an electrical part) and a master cylinder failure typically doesn't exhibit just a soft pedal, I would say it isn't the master cylinder. From experience, I too recommend a new master cylinder rather than a rebuilt. If you can answer a few questions it could give us a better shot at helping you. Did you bench bleed the master cylinder when you installed it? Have the calipers been rebuilt or are they new? If new, are they new castings, rebuilt, sleeved, O-ring or original seals? Is there any evidence of a leak anywhere including the connector blocks? Have the rotors been changed and are they in spec? Are the wheel bearings good, especially the rear? Have the lines been replaced? Have the brake hoses been replaced and if so, are they original or covered with braid? Are the pads new or worn? Is the pad wear pattern even? Is there a hiss from under the dash when the car is running or a whoose when you press on the brake? I have been where you are and about came to the end of my rope. So far, the upgrades I made are holding but it has been a long and strange journey. If you are still having problems, give me answers the questions and I will try to give you my two cents.

jrs

 

 

ok well i'm checking the callipers today. I opened the mastercylinder to find that there's no fluid in the front and half in the back. The car was sitting over the weekend having the transmission rebuild. Yes, i bled the brakes. The brakes worked fine before i dropped it off. I always seem to notice some brake fluid on the bottom of the brake booster though. I believe the brake lines are original and i doubt they've been inspected.

 

Sounds a little like a problem I gad earlier this year. Had to bench bleed the !@#$ out of the master cylinder then pulled about 3 qts thgrough the calipers. I have a good solid pedle now. My $.02

 

A little off topic but....
What brand of brake fluid are you using?

 

-that raises a red-flag to me, does that enterprising Transmission-shop also do brake-jobs? Seems rather suspecious, that the brakes were okay when you left the car there-- No? Try refilling the MasterCyl. and check to see if the pedal pumps-up and holds-solid (bleed-air in lines if necessary), then drive as usual to see if it stays fixed; --if so, then you need to be angry as hell, and report this repair-shop to the proper Consumer-agency, -as tampering with a customer's brakes in order to drum-up business is an all to common life-threatening violation of the law...!! Since brake-fluid was mysteriously missing (although you did say you have detected some fluid-seep'ege), -the soft-pedal condition is thus not a problem that could be attributed to old wheel-cylinders lip-seals pumping-air, --but rather is air being introduced via the empty MC. Please let us know of the outcome of this issue....
~Robair vH

 

That much fluid loss that quickly should leave tell tale signs somewhere. I will be interested to hear what happens when you fill the reservoirs and bleed the system again. When you do that, be sure to clean off everything around the master cylinder so you can see any leaks in that area. It still seems odd that you see brake fluid at the bottom of the brake booster. There are only two ways I can think of that would cause that, a leak where the lines go into the master cylinder or a leak in the seal around the piston. In my estimate, both of those are low probability failures but stranger things have happened. Keep us informed.

 

Assuming you bench bled the M/C(s) before install and since you're loosing that much fluid it isn't a minor leak, check for a leak at the calipers...my fronts have a very, very slight leak and the plastic on the back of the pads is wrinked and warped from the brake fluid..I'd pull all the pads and look for leaks..you'll know it if the pistons are leaking...check for fluid on the inside of the tires first, that will indicate the bad caliper(s) (with that much fluid lose, there should be all kinds of fluid someplace, tires, floor, frame)..may save yourself some time...if that is ok, follow the lines back to the M/c for leaks....and this "..notice some brake fluid on the bottom of the brake booster.." sure sounds suspicious, if it's coming or came in from the back of the M/C..you need to know why, may be just left over fluid from the first bad M/C that had a bad diaphram...and just a final question, when you replaced the first new M/C, DID you notice fluid on the back of the M/C where it mates with the booster, or if you pull it now, is there fluid in there?

 

Good old Advance Auto DOT 3.

 

yep...you just said it.