Bad Master Cylinder?

AHartz · 05-01-2011, 02:33 PM

I've been fighting a soft brake pedal for some time now. While bleeding the brakes again today, I thought I was making progress and getting periodic air bubbles to flow from the left rear wheel caliper. I have the speed bleeders and I was using a motive power bleeder. When I got the bubbles to stop flowing, I closed the bleed screw and removed the power bleeder. I then opened the bleed screw again and did a few pumps with the brake pedal (short strokes - not to full lenght of pedal travel). After closing the screw again, I went to check the pedal firmness (which had seemed to be getting harder), and the pedal went straight to the floor! I double checked that the bleed nipple was closed and didn't see any other fluid leaks. I had been suspecting that the master cylinder was bad and now am going to buy a new one.

I've been trying to fix the brake system for a while now. Over the past two years, I first installed a new master cylinder, then new flexible lines, rebuilt the calipers, installed new pads, and the speed bleeder nipples. I must have bled the system with six quarts of fluid in attempt to get a firm pedal following all the tips I have learned from this board.

Since the system is closed, am I correct in thinking that as long as I'm not leaking fluid anywhere, the only reason the pedal would go to the floor with no resistance is that the master cylinder is bad?

Adrian

Joe Bob · Joe Bob's Garage

Keep bleeding and tap the calipers with a rubber mallet to free bubbles. Use an atlternate colored fluid to know you have run the line complete. Wait a day or two and do it again.

I assume the bleeders are on top?

crashr · 05-01-2011, 03:05 PM

This sounds exactly with like the problem I have. I have bled my brakes several times, and still didn't get the pedal firmness that I had expected. I then changed the master cylinder which helped only slightly. Can a sticking caliper piston give this kind of soft pedal feel?

Joe Bob · Joe Bob's Garage

Yes and old rubber brake lines that are swollen on the inside. They would cause a caliper to stick.

Lines are cheap and an easy replacement. If they haven't been done in over ten years, probably a good time to do so.

Fluid? When was the last a flush was done? Pulling the calipers and doing a visual couldn't hurt.....

AHartz · 05-01-2011, 03:16 PM

I have been driving the car with a soft pedal, up until this weekend (can stop the car, but pedal is mushy). I thought I would give it one more try of bleeding the brakes this weekend before I purchased a new master cylinder. But halfway through the job, it seems that the master cylinder has given out and I no longer have any pressure at the pedal.

Yes, I've been banging on the calipers to loosen bubbles, and the bleed nipples are towards the top.

A month or so ago, I installed new pads, and bumped the pistons out to make sure the pistons were engaging with the pads. The pedal was relatively high, just mushy.

Joe Bob · Joe Bob's Garage

How old are the rubber lines? Cheap easy r&r......not like it will screw up the brakes. I don't like s/s lines as they can fail catastophically......

$40 ish and piece of mind.

wildcat077 · wildcat077's Garage

Thing is when you're doing a complete brake flush or installing new calipers,it's best to use a pressure bleeder such as a Motive so you don't overextend the internals in the master cylinder by pumping the brake pedal, wether it be a new one and even more importantly an older one.

Once you have enough pressure it's OK to bleed by pumping the pedal but a pressure bleeder is still the best tool for the job.
I did a complete brake makover on my car,23mm master,996TT calipers which have two bleeders per caliper,new hard lines,new SS lines and had no pedal issues whatsoever but i did use a Motive right from the start ...
It's worth the try as a last resort !

Cheers !
Phil

AHartz · 05-01-2011, 05:23 PM

Thanks for the replies.

I haven't seen the inside of a master cylinder, but I suspect there is at least a seal near the outboard end (the non-closed end towards the vacuum booster) and a few seals that create separate "chambers" for the front and rear brake systems. If this is true, then if the outboard seal fails, it will leak into the vacuum booster. If one of the internal seals fail, the fluid just moves inside the master cylinder. Does this sound correct? I'm just curious why I have no pedal pressure, but don't see any leaking fluid either.

gregwils · gregwils's Garage

Here is a great master cylinder thread. I rebuilt all my calipers, replaced all the flexible lines and replaced the master cylinder a few years ago as a precaution since my car was new to me. It takes some time to complete, but you can't mess with bad brakes...or any safety item.

I think the fluid just leaks past the seals in the MC relieving the pressure, which is why it isn't forced through the lines. It doesn't really go anywhere, that's the problem.

You don't need a pressure bleeder, although wildcat's advice should be followed. You don't want to bury the pedal to the floor, you could possibly damage the MC seals. I have speed bleeders and pump the pedal by hand so I can control the distance I am depressing the pedal. I pump in short strokes numerous times, rather than one big long plunge. You must have speed bleeders to pump in short strokes.

Rotate between yellow and blue fluid every year. Get a set of speed bleeders even if you decide to buy a pressure bleeder. Also, bleed from farthest caliper to closest - RR, LR, LF, RF, LF.

If I do a thorough job bleeding, my pedal is rock hard. Good luck.