master cyl wont release

[poorbiker]

Well got my buddies 69 going but encountered an issue. We scored a very good looking oem master cyl and installed a new kit, bore looked very good, maybe to good since it was of course used. Rebuilt many cyl with no issues. Problem is after applying brakes a few times while riding brakes lock up tight, have to bleed off pressure at mater cyl brake line to release pressure.
Took it apart and cant find anything installed wrong, installed another kit and same result.
I mentioned the bore, if someone had honed it to much would or could it cause the plunger to lock up in the bore.......

[sirhrmechanic]

could the rubber be getting caught in a gap created by too large a clearance between the piston/plunger and the bore? Could fluid be getting past and then locking up both sides?

We have our cylinders sleeved and bored by White Post Restorations in Virginia. They will bore over and put a thin brass sleeve inside, restoring clearances. Rapid turnaround, great work. That said, may not be cost-effective for a Shovel cylinder. We use them for antique car work, though, and they are first rate.

Just some thoughts. Hope this helps.

Cheers,

Sirhr

[dane]

could the rubber be getting caught in a gap created by too large a clearance between the piston/plunger and the bore? Could fluid be getting past and then locking up both sides?

We have our cylinders sleeved and bored by White Post Restorations in Virginia. They will bore over and put a thin brass sleeve inside, restoring clearances. Rapid turnaround, great work. That said, may not be cost-effective for a Shovel cylinder. We use them for antique car work, though, and they are first rate.

Just some thoughts. Hope this helps.

Cheers,

Sirhr

Plugged bleed hole in cylinder. Won't allow fluid to return back to cylinder.

[Flash92]

Maybe the caliper piston isn't moving freely? Any moisture in system will turn to steam & expand holding pressure on the system when heated up during use too.

[rousseau]

make sure you have enough "freeplay" between the brake pedal and and plunger rod.

[poorbiker]

I have heard of more than a few guys sleeving these old master cylinders, have not considered trying it.

Drum rear brake.

Checked for plugged bleed hole, nothing found.

The bore could be to big, someone could have honed the crap out of it to get out some pits or scoring and it never worked for them ending up on ebay, I dont have an inside cyl mic that small, hmmm. Guess I will have to get another.................

[poorbiker]

Do you mean between the plunger rod and plunger, hmmm, will go out and check that for not enough clearance.......

[dane]

Plugged bleeder hole may be hard to see as it is very small. You said you have relieved pressure by loosening fitting on master cylinder. Obviously the fluid is not being allowed to return to the cylinder and is being locked in the system.

[poorbiker]

Well checked clearance, this setup is using a fixed plunger rod (non adjustable) with a modified brake pedal setup and it does appear there might not be enough if any clearance between rod and plunger. Problem occurs only after bike is good and hot, Im guessing heating and expanding all parts enough to keep the plunger from fully releasing.
Will fix this and put it to the test tomorrow.

[poorbiker]

Dane,
Thats what I thought also, I have some torch tip cleaners I used to make sure hole was clear, it was, is. Washed out thoroughly. I will check it again though, cant hurt, like you said something is blocking hole or it should relieve pressure, thnx.........

[Jack K]

Some may be getting away with it but it's been my experience if that little master cylinder has been honed, the cup will drag and not return reliably. The bore has to be very, very slick. Like polished tool steel. The only way to make it work as it should is to over bore it and press in a polished sleeve. I think the sleeve I used was marine brass. Just my experience.

[Flash92]

Maybe the caliper piston isn't moving freely?

Sorry, I had meant to say the piston in the rear wheel cylinder may not be freely moving.

[BigLakeBob]

I believe stock bore is .755.

Jack K, what is marine brass?

[sirhrmechanic]

I believe stock bore is .755.

Jack K, what is marine brass?

Common term for a very high-grade of machinable brass. Generally harder than a decorative or dead soft brass. Often specified for Naval ship parts and components... because it's makes for very nicely machined/cast bits but will also never rust. I think it encompasses several different alloys, being something of a generic term.

Cheers,

Sirhr

[vph-d]

It seems to me there was a check valve included in master cylinder kits since they fit both disc and drum brakes. I don't remember if you were to leave it out for drum brakes, though.
VPH-D