Master cylinder return spring update...I don't get it

It was the seals. The ones in the MC I'd bought for a mid 60's Chevy truck were not compatible with mineral oil brake fluid and had swollen up.

So I still had the 8700's original master cylinder on the shelf with the aftermarket rebuild kit still in it. The rebuild kit was marketed for Ford 8700 rebuilds so should be compatible with mineral oil brake fluid. The cups didn't look swollen/soft after a year of residual mineral oil sitting in the bores. It had leaked in the original MC when I put the kit in, I blamed it on the severely pitted cylinder bores.

So took the rebuild kit out of the old MC and put it in the new MC. If anybody local would have had a new rebuild kit I'd have bought it, but nobody did, sooo...Actuated the pistons while on the bench, free as can be. Did stretch the springs 1/2" just to do it.

Put it in the tractor, went to bleed the brakes...and after getting fully depressed the pistons don't want to return! They do, eventually, but very slowly. Took me forever to bleed the tractor. Now that I've got the air out of everything, I'll get maybe 1/2" travel on the pistons before they won't go down any more.

The picture is of the old MC, which is the same dimensionally as the new MC, just with rust. Are those little bitty holes what let the brake fluid back in from the reservoir? Is that why the pistons were so slow to return, trying to get fluid through those holes? What are the bigger holes for? I need somebody to school me on this. Swapped out a lot of MC's on cars over the years, never had these issues. Just like to know what's going on.

Thanks

Fred

The little holes are what let the fluid back into the reservoir when the pedal is released.

When the piston fails to fully return, the little hole remains covered, and the line remains pressurized, the brakes don't release, the fluid heats up, applying the brakes even harder.

The bigger hole is where the fluid gets from the reservoir into the cylinder, ready to send to the wheel cylinder when the pedal is pressed.

When installing a new/rebuilt/empty master cylinder, it's always best to "bench bleed" the cylinder. Fill it with fluid, hold your fingers over the line ports, push the piston in using your fingers as check valves to expel air and draw fluid in. Keep pumping and watching for bubbles coming in the reservoir and air coming out the ports. When it is free of air, install it quickly so it doesn't leak down or get air in the cylinder.

That way you don't pump air in the system, usually can just bleed it at the lines.

Good that you got it going! I suspect it will be good to go now.

Tiny one called compensating port (from my SAE training in 1968) Jim

In the static position, the compensating port also allows for fluid expansion due to heat. Without the port, heat will expand the fluid and pressurize the brakes.

I would buy another kit and rebuild that old mc. Use a brake cylinder hone to clean up the bore. Use brake fluid while honing and keep it moving until the pits are gone. The old type seals can handle a lot of honing and still work fine. I have cleaned up a lot of pitted bores that way.

With 0 rings you would have to have it bored and sleeved or go up a size on the piston and seals.

I tried bench bleeding last year when I rebuilt the original MC

But the tractor is a Ford 8700 with a cab, and the contortions you go through getting the MC bolted back up to the firewall...lost all the fluid anyway.

IF I have to do it again (please no) I'll try bench bleeding, get some plugs for the line ports and use the solid reservoir cover that came with the new MC as opposed to the one with the dipstick tube that the tractor uses, then swap out the covers when it's bolted up. Live and learn.

But today I've got good brakes, they release just fine, and I'm happy

Thanks for the help

I honed on that thing forever last year, the pits were just too deep. It had been neglected for years before I got it, the rust was severe.

The good thing learned, at least for this tractor, is buy the MC in the link below. Then remove all the non mineral oil compatible guts and replace with a compatible rebuild kit.

Basically winding up with a new MC close to $600 cheaper than what New Holland gets for it.

Thanks

Fred
1966 Chevy C10 Master Cylinder

(quoted from post at 18:47:57 05/29/19) I honed on that thing forever last year, the pits were just too deep. It had been neglected for years before I got it, the rust was severe.

The good thing learned, at least for this tractor, is buy the MC in the link below. Then remove all the non mineral oil compatible guts and replace with a compatible rebuild kit.

Basically winding up with a new MC close to $600 cheaper than what New Holland gets for it.

Thanks

Fred
1966 Chevy C10 Master Cylinder

Click to expand...

pple Hydraulics in NY will bore and sleeve with brass and put end to rusty bores. Did that on my '62 Chev C10 hyd clutch slave cylinder.