Master cylinder return spring

Ford 8700 dual master cylinder, pistons won't return all the way to allow brakes to release.

This is a new last year MC I bought last year. Ok, it was for a mid 60's Chevy truck I bought for $40 off of Amazon, New Holland wanted $700 for a MC. Had tried rebuilding my original, but too much rust pitting in the bores.

Anyway, when I was bench bleeding the new one it seemed to hang up about 3/8" from full return. Worked it some, and problem went away. Used it last summer with no problems.

Get the tractor out to bale hay this year, right piston is hanging up again, wont allow brake to release.

So my question...do I remove the spring, stretch it out and re-install it? Will it stay stronger? Or do I buy different stouter springs and install them?

The spring that's in there is 3.5" long, 3/4" diameter, 0.060 wire diameter. Can get springs with the same dimensions except 0.080 wire, may have to cut a couple coils so it will depress all the way.

What would you do?

As a side note...when I tried rebuilding the original MC, it was hanging up also. I blamed it on the bores, but I'm thinking now it may be the pistons they use now ...Orings on the pushrod side of the piston vs a cup on the original
MC.

Pics are of the MC I bought, and of a rebuild kit for that MC

Thanks

Fred

If it worked last year it must be close to working now. I bet by stretching the springs would be enough.

Do you know the history on the reactor?

Any chance oil has been put in the system?

That is a common cause for MC to not return. It swells the seals, makes them too tight in the bore.

I would go in and take a look at the seals. If any sign of swelling, replace the seals, including the wheel cylinder seals, replace any rubber lines. Flush the system thoroughly with alcohol based cleaner, brake cleaner, nothing petroleum based!

That is a worst case situation. If you are sure no oil is in the system, then try modifying the spring. Do you have the spring out of the old cylinder? See if it may have been stronger.

That should be tractor, not reactor! LOL

Stretch the spring about 1/2" and give it a try. There is no safety concern as there would be in a single bore dual circuit MC. so a stiffer spring is no issue. Make sure the pedal and linkage is not catching in some way, or the pedal return spring is not strong enough. (it happened on both old. and new, unlikely) Jim

Hmmm...now that you mention it...

The 8700 uses a mineral oil brake fluid, not DOT3.

The MC I bought was for a mid 60's Chevy truck, Chevy used one side for the brakes, the other for the clutch. I remember wondering at the time (and just remembered it now) if the seals were compatible with a mineral oil brake fluid.

Could be

Fred

And I was all set to check the flux capacitor for resistance

Fred

Automotive master cylinders are not comparable with mineral oil, only DOT 3. Mineral oil will cause swelling and component sticking.

Are there any flex hose before the wheel cylinders? I am not familiar with the setup on your tractor but it is common in cars and truck to have the front brakes hang up because the hose has collapsed inside and won't let the fluid return. Are there any springs on the brakes that open the wheel cylinders?

Nope, just steel tubing.

Do not use DOT3 brake fluid it will swell seals on brake pistons too.

Well, the dealer may have you right where he wants you!

Might try locating a master cylinder rebuilder, see if they have a source for oil compatible cups and u cups.

I've never tried to find them, I hear some foreign cars use mineral oil, but they would likely be metric.