Master Cylinder help needed

[ChilinMichael]

I think my master cylinder is going. The car gets it's brakes bled annually, and after new brakes the car just doesn't feel as it should, it was my mechanics advice that the master cylinder might be the culprit in this. How can I check further? Also...I see there are numerous "size" master cylinders, what do I want/need?

[RJolly87]

How do the brakes feel now, causing concern?

And the size is determined by measuring the length of the unit, mounting flange excluded I believe. It is going to be obvious when you put a tape measure on it.

[ChilinMichael]

It was before, whenever I stopped at the very end of the stop where it's coming to 0, it would get very soft but that seems to have alleviated itself. It happened only after compressing the pistons on the calipers to install the new pads. Also, I have taken notice the sliders were frozen cause only the outer pads were worn down, not the inner. I have greased the sliders but will re-check in a week.


Before this, I had felt varying brake feel, like sometimes it would go down further on the pedal than others, but if the sliders for the calipers weren't working right....I guess that could cause a lot?

[repinS]

Yeah, I would be wary of the master cylinder. When mine went, it would only start losing pressure in one of two ways:

1.) Every dozen or so brake pedal presses
2.) Pedal would go to floor after holding brakes on for 5-10 seconds

#2 could be your symptom with the softness when coming to a stop.

[ChilinMichael]

It seemed consistent tonight after it's little episode, I'm gonna keep an eye on it, but I know it has a LOT of miles on it more than likely, so I might be changing it very soon....

[repinS]

There's also the dribble of brake fluid on the outside of the brake booster below the MC. I actually missed it the first time I was checking, fluid level looked good at the time and it was a small dribble.

[ChilinMichael]

I'll clean up tomorrow and take a look after, since today I compressed the pistons on the calipers there's fluid that leaked out of the tank so yeah, kinda hard to tell now.

[spoiledred94]

I was told my brakes could be suffering from a bad BMC, but my symptoms don't match.

I changed the brakes and 1 rotor on my friends van and when I compressed the caliper brake fluid squirted out the two tiny vent holes on the BMC cap.

[RJolly87]

That is normal when you compress the caliper, but I am surprised that you didn't replace both rotors. My master cylinder when it was going both times was indicated by a low and spongy pedal, even after bleeding and adjustment.

[bill buttermore]

Sometimes, master cylinders need to be replaced a few months after the front pads are changed. Here is one way that can happen, and how you can avoid it.

Heavy, abrasive contaminants collect in the brake fluid at the lowest point in the system, i.e., behind the caliper piston. When the caliper pistons are compressed to change the pads, this contaminated brake fluid is forced through the master cylinder into the reservoir. These contaminants can quickly damage the seals on the piston in the master cylinder causing the seals to leak and allowing the pedal to slowly drop to the floor when the brakes are applied.

To avoid this problem, open the bleeder screw on the caliper while compressing the caliper piston. The dirty fluid can thus be diverted to a pan, and the master cylinder will not overflow. If you are careful to only open the bleeder while pressure is being applied and to close it when the piston bottoms out before pressure is released, air will not be sucked into the line and bleeding will not be needed.

Periodic flushing of brake fluid will remove these contaminants and minimize the probability that you will suffer from this problem.

[ChilinMichael]

I usually flush annually, but I noticed the fluid is dirty. I may do a flush again and clean the system out well due to how much AMSOIL Dot 3 high performance fluid I have and it's low cost to me. The car is being very consistent now in braking, I am thinking it needs a flush....

[wpbrink]

Just curious my master cylinder went out and will be replacing it. Since air is in the system do I need to bleed the ABS system also? If so how do you do it?