Brake bleeding ?

[boatpuller]

Installed my rear disc brakes this weekend, all new rubber lines,new front pads, rotors turned. Brakes had been getting kind of spongy in the last month or so, when I started bleeding the brakes couldn't get the pedal to stay up, it would go to floor, so went down and bought a brand new master cylinder(Raybestos) bled on bench, installed, rebled, starting with antilock valve then all wheel cylinders, pedal got hard, felt solid until I started engine, then pedal slowly goes to floor, even pumping it up doesn't help it goes to floor after a couple seconds. Have gravity bled, bled the old fashined way and vacuum bled. I even loosened brakeline at master cylinder with my better half(she might read this) holding pedal, if pedal was on floor would still get pressure out of MC on front port(for RR brakes) but wouldn't out of rear port. Could only get fluid out of rear port(for FT brakes)in the first amount of time after she pumped them up before it went to floor.

Here's my question, do you think that the fluid could be going by the piston for the front brakes(rear piston/reservoir) only when I have the extra assist from the booster?? If not do you have any other ideas, one other thing, when you push on the pedal with the reservoir cap off the front reservoir spits up at you but the rear one just barely moves. I'm going to see about getting another new MC, just wondering if there isn't something else I should look at also? On my dirt driveway the rear wheels will lock up but the fronts don't. Also put vacuum gauge on booster and the pump is putting out plenty of vacuum.

Thanks for any suggestions,

Caleb

 

[CDegner]

Sounds like air in the front. When you bled the front, did you do passenger side first then driver side?



Then again, it wouldn't be the first time I have heard of someone buying a rebuilt MC and t being bad. I will not buy another rebuilt master cylinder unless there is no way I can find a new one. I have had 4 of them go bad on me.



Carl

 

[boatpuller]

Yes, bled antilock valve, right rear, left rear, right front, left front in that order(probably 10 times, a couple with vacuum bleeder, the rest with help from my wife). If I had air in the front wouldn't the pedal be soft without the engine running?

It is a "new" MC not rebuilt.



Thanks,

Caleb

 

[62MaxWedge]

You have air trapped in the anti-loc valve, it is REAL hard to get it out.



Open the valve, and use a vacuum pump just watch it, you will be amazed at what you see.



This takes time and patience.

 

[dan brooks]

Make sure the pushrod length is properly adjusted. After replacing my booster we had to tinker with it because the rear brakes were coming on way too soon (shortening the rod a few turns helped tremendously). I don't have my FSM with me, but maybe someone here can post the proper procedure to adjust it.

 

[dpuckett]

Ditto on the RWAL valve; they are a real PITA, not to mention they reduce brakeing performance. Now, on to bleeding. Are you using a clear rubber tube attached to the bleeder screw with the other end submerged in brake fluid? Do that, then continue with the 2man operation- build up pressure in the pedal, crack the bleeder, once pedal falls, close bleeder. Repeat till no air is present for 5-8 cycles. With the hose attached, it is nearly IMPOSSIBLE to get air in the system. Once you are certain you have no air in the system, and you still have the pedal falling sysndrome, I'd guess you got a bad unit. On my 92 W250 2 years ago, I went through 4 NEW master cylinders before i had one that was any good.



Also, how are you bench bleeding? If you are using the rubber tubes going from the ports back to the reservoir, you may be reintroducing air into the system. The last MC I got had instructions with littel plastic plugs to screw into the ports, then you push the rod about 3/4-1" in, and bubbles come up that way. Cylinder MUST be level for this to work. Much faster, and in my experience, better way to do it.



Only other thing I can think of is to bypass the RWAL valve and have full rear brake function.



Daniel

 

[boatpuller]

Bled everything a couple more times(I have used a gallon of brake fluid bleeding so far ) then decided to put my original master cylinder back on just to see what happens, didn't bleed anything except for the MC before putting it back on, still a little mushy but way better without even bleeding than the "new" Mastercylinder. I will be returning the "new" one tomorrow for a new "new "one.

Thanks for all the suggestions,

Caleb