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I have had this problem with my M37 brakes for quite a long time. If it sits for a few days I find spongy brake pedal and low fluid level in the MC. I power bleed using Motive pump type bleeder and get a hard pedal. A few days later the same thing. Over the course of time I have replaced the following; MC, stop light switch, all brake lines and all brake hoses. I have also tried closing the vent line petcock. There are no visible signs of leak anywhere. Each of the many times I have bled the brakes I do get some air bubbles from one or two of the bleeders. The only component that I have not replaced is the wheel cylinders. Is it possible that one of these could cause this type problem?

If you inspect them by pulling off your drums there would be visible brake fluid inside the drums and you may have saturated the brake linings. I would call them suspect, after all what else is there left?

I'm with Tuko pull the drums and inspect. there has to be a major leak to show wetness on the backing plates and only after pulling the dust boots off the cylinders will you know if there is a leak. often scum and nasties will be present at suspect cups, pistons and bores. knock off scum in bore with hone. clean everything with alcohol replace any cup that has even the slightest scratch scoring or pitting(cups are >$5 each). wile your in there inspect and free off any stuck break shoe cam (major and minor).

GOOD MORNING, I have/had a similar problem, it was the right rear wheel cylinder; but I saw a wet area on the inside of the plate before so I knew where to look.
BOB

xd2200:

The "disappearing" brake fluid has to be going somewhere. The MC and wheel cylinders are the prime suspects but if
that is not the case, you should inspect every inch of steel and rubber brake line. You may have a pin hole in a line or
a loose fitting and it may be hard to see. Use good lighting and a clean shop towel to wipe down the lines and look for
signs of moisture and scum.
Let us all know if and when you discover a "fix."
Jerry

THANKS to all that commented. As suggested I pulled the rear drums and sure enough one of the wheel cylinders had a leak - luckily it was small enough not to saturate the linings. I will pull the two front drums and rebuild all four cylinders. Again, thanks for the help!

Just went thru my brake overhaul and the RR wheel cylinder was the leaker. Glad to see some of our other M37 vets weigh in!
Gary

I have a question , is changing the front wheel cylinder as easy as it is do to the rear? I just replaced my two rears, id like to try the front next, Jay

Hi Jay,
It's not any more difficult than the rear WCs.
Gary

Thanks, I was worried id have to pull more items since its the front.

Don't forget that when inspecting or working on the braking system, you don't have to pull the whole hub, just remove the three flathead screws between the wheel studs and the drum comes off separately.

Sure saves a lot of time and trouble .

Unless you need to repack the bearings anyway .

DG

Rebuilding a M/C or wheel cylinders is a total waste of your time. Very likely they will be leaking again real soon. The truth is this; practically all cylinder bores will have rust pits in the walls; by the time you hone to the point of getting rid of all pits, the bores will be too large to hold a new cup for long; and if you don't remove ALL the pits, you really have wasted your time and $$. New cylinders are available at a reasonable cost; that is by far the better way to go. The time you will save will more than pay for the new cylinders; and you won't need to do it again in 3 months.

just a tip on replacing: if you have cast iron pistons in your original wheel cylinders clean them up and install in the new wheel cylinders. None of the new cylinders I have seen have cast iron they are all aluminum. the aluminum causes corrosion and very premature failure of the rubber cup. unlike metals don't like to be next to each other