M38 Master Cylinder

[Richard Whelan]

Anyone out there know how to diagnose a master cylinder problem on my M38.
It is new. The piston would not return properly when I first bench tested it on my vice. It was well lubricated but would not work properly. So I honed out the bore until the piston moved freely. Bench test it again with brake fluid and no leaks were found so I installed it and started to bleed my brakes.

Now here is where the problem begins. When I bleed a wheel cylinder the piston takes about 2 minutes to return fully to the rear. I think there is some sort of vacuum problem but do not where to look. I think it is still in the master cylinder but not sure what to do about it. I've checked the intake and bypass holes and they are clear.

Note. When the piston finally returns fully the master cylinder functions correctly but the brakes are still spongy. So with the problem when bleeding a wheel cylinder this prevents me from removing all the air.

Completely confused

[rob love]

Sounds like you got a bad one right out of the box. Any chance you damaged the cups by over-travelling the piston into the master cylinder?

Make sure you have free play on the rod to the master cylinder. Mind you that will normally lock up the brakes rather than cause slow return.

Personally, I use a bleeder ball to bleed brakes as opposed to using the master cylinder to create pressure. Alternatively, you can also use a vacuum pump at each bleeder screw....the pumps are available relatively inexpensively.

Master cylinders are readily available and cheap. I see on ebay where they are listed at $35USD.....worst case is NAPA where they are $100 CAD.

[mel gabel]

I rebuilt the master cylinder on my M38 and then proceeded to try and bleed the brakes. I encountered problems similar to what you describe and discovered that I had blockage in the flexible line between the frame and the rear diff. Replaced that line and the bleeding process went smoothly. The problem was not with the master cyl at all.

[Richard Whelan]

I cannot confirm any damage to the cups but they do look ok and it did bench test ok. I have been thinking it is a bad master cylinder but wanted to have some other opinions in case I was the problem. A friend of mine says he's never seen one like mine. It will probably be cheaper to purchase a new master cylinder but I will check my flex lines to make sure they are not the problem first.

Thanks Gents

[Bob Carriere]

making sure they are disconnected at both ends.............. not a bad idea to introduce some ethyl alcohol into the lines as a flushing agent...... large hypodermic needles are available cheap at Tractor supply....... then blow the line dry before reconnecting.

Not sure on the M38 Master cylinder.....but on CMP and other older GM models they have a tiny, needle size, bleedhole in the bottom of the MC...... you returning piston, after pumping, must clear that hole for the MC to work properly...... CMP MC habe a screwed adjustment on the push rod that fits into the MC..... playing with this adjustment will allow the piston to clear the pinhole in the floor of the MC.......PIN HOLE is visible from the filler opening.

Good luck.

[Richard Whelan]

I should've mentioned I installed new lines and flex lines last Fall. Since I made all the lines I will confirm no lines and flex lines are crushed or have some sort of debris inside just to be sure.

As well all my brakes pads and wheel cylinders are also new.

Don't want to purchase a new master cylinder just to find out one of the problems pointed out here was the problem.

Thanks

[David Herbert]

If the master cylinder does not return after pushing on the brake and is all connected up and blead as well as possible you could try pushing the brake and while it is trying to return (might take two people- one with a spanner one pressing the brake) crack open the hydraulic fitting on the outlet of the master cylinder. If it snaps back that would suggest a pipe blockage - usually a flex hose with no hole up the middle because the rubber has swelled. Enough pressure when pressing the pedal but not enough from the return springs. Also crushed metal pipes do happen.

David

[Bob Carriere]

Hi Richard.

On the outside chance that one of the newline or flex line is plugged..... I would still recommend a good blowing od all lines...... then you can focus on the MC..... Eastwood auto parts even sell a special can of ethly, pressurized like a paint can, to flush out new lines of any manufacturing left over oil..... or well hidden gift from a mud wasp!!!!!!

Good luck and keep us posted of your eventual solution.