getting a disc brake to pump up

An update - after waaay too many hours of fruitless bleeding on this silly thing it still won't pump up. Here's what we've tried..

We noticed that the pistons in the caliper would move outward but spring back by themselves. It seems they are being held captive by the new rubber seals and won't slide out against the rotor. The original caliper had locked up and got really hot, so he decided to refurbish a spare that he had with new seals and stainless pistons. If the pistons don't come out far enough, it'll never pump up tp pressure. At least, that's my logic..

Soo... we put the old caliper on. Still won't pump up.

The MC was sleeved years ago by some guy in Massachusetts. All we did was replace the 2 piston seals.

We've bled the system both on and off the bike. Best results so far was using a syringe to force fluid into the caliper bleeder. Something didn't seem right though... at some point the fluid stopped flowing to the MC and would just leak out around the bleed screw. It seemed like something was stopping fluid from entering the system.

Very frustrated at this point. I just dunno what to do next.

I have stainless pistons an a sleeved master cylinder. Done by RGM. I notice one of the pistons springs back about 10 or 20 thuo this has happened to me on cars and bikes in the past when new seals were fitted. in time the seal relaxes an the piston stops doing it. mine has done 300 miles an is still doing it but the seal is a square section so doesn't matter how it is fitted it is the groove in the calipar that has the angle to help the seal hold against the pressure.
Give it time an ride carefully it should behave sooner or later.

This worked for me a few times.

1. Bleed the brakes (either the old way or using a vacuum bleeder. Don't expect it to be perfect. If you can pull it back to the bar that's ok. Just try to get a majority of the bubbles out.
2. Clamp lever back for several days. Don't touch it.
3. Loosen clamp (after several days, right?) and check. I bet you'll have nice brakes.

Report back.

If you are pulling air past the bleeder screw you are loosening it too much. Just needs a tiny crack to bleed.

I find if you remove the bleed nipple and put a SMALL smear of the red SPECIAL TYPE brake grease on threads it normally stops air going past the threads unless they are very loose fit in the calipar. Also next time you try to loosen it in the future it helps stop it seizing in but the best way to stop that is fit a stainless one.
But I stress use a SMALL SMEAR OF THE RED SPECIAL TYPE BRAKE GREASE.

maylar said:

The MC was sleeved years ago by some guy in Massachusetts. All we did was replace the 2 piston seals.

We've bled the system both on and off the bike. Best results so far was using a syringe to force fluid into the caliper bleeder. Something didn't seem right though... at some point the fluid stopped flowing to the MC and would just leak out around the bleed screw. It seemed like something was stopping fluid from entering the system.

Very frustrated at this point. I just dunno what to do next.

Click to expand...

When you say it won't pump up do you mean that there's no resistance at the lever at all? Right to the grip each time?

I had the hydraulic clutch on my VFR do that after sitting over the winter. Replaced all the rubber parts in the M/C and slave. No change. Took the M/C apart and checked the little holes. You could see light in the big one but nothing on the smaller one. I took a strand from a plastic brush and ran it through the smaller hole, then brake cleaner and air. Fixed it. If it's been sleeved it might not just be crud, sleeve moved? The M/C has worked after sleeving? I would think not pumping up at all would have to be a M/C issue. Maybe not?

Maylar, from what you're saying it sounds like (using the syringe) you were trying to push fluid from the caliper up into the master cylinder? I don't think it would work that way around due to the "cupped" shape of the master cylinder seal. I think you should be using the syringe to draw fluid from the master cylinder down into the caliper, hope this makes sense to you,

Dave.

daveparry said:

Maylar, from what you're saying it sounds like (using the syringe) you were trying to push fluid from the caliper up into the master cylinder? I don't think it would work that way around due to the "cupped" shape of the master cylinder seal. I think you should be using the syringe to draw fluid from the master cylinder down into the caliper, hope this makes sense to you,

Dave.

Click to expand...

Pressure bleeding is very effective. I use a master cylinder reservoir cover with a hose nipple attached and a small weed sprayer with a pint or so of brake fluid. Pump up the sprayer, connect the sprayer to the nipple, lock back the trigger so there is fluid pressure and crack the bleed nipple. Works better than any other method I've tried.

Dave,
This doesn't look anything like a Lockheed M/C but the main parts are there.
You might want to try using some teflon tape on the threads of the bleeder and only open it enough to lift it off its seat to pass fluid. If you can't get fluid into the reservoir by pumping it into the bleeder on the caliper then you've probably found your problem. Something is blocking the compensating port. Could be the piston isn't retracting all the way, the sleeve is misaligned and blocking the hole, the guy forgot to drill the hole for the compensating port? Maybe the dimensions of the piston are off. It would be nice if you had another M/C that you could use to compare your stuff to stock.

This is a link to a gif on Wikipedia. The names of the ports look like they are mixed up. You can see that there shouldn't be anything in the way of fluid moving from the caliper and the reservoir in the M/C.

http://en.wikipedia.org/wiki/File:Hydra ... iagram.gif

Jimc . yes the syringe method pushes fluid up the system from the bleed nipple. Air bubbles and fluid come out of master cylinder piston via the vent holes in fluid reservoir. When no pressure is placed on piston via the brake lever these holes line up to let fluid into piston and any pressure in system caused by build up of heat in calipar to vent so brake does not drag. When you move lever and piston starts to move it covers these holes so pressure is created and forced down brake line to calipar to

toppy said:

Jimc . yes the syringe method pushes fluid up the system from the bleed nipple. Air bubbles and fluid come out of master cylinder piston via the vent holes in fluid reservoir. When no pressure is placed on piston via the brake lever these holes line up to let fluid into piston and any pressure in system caused by build up of heat in calipar to vent so brake does not drag. When you move lever and piston starts to move it covers these holes so pressure is created and forced down brake line to calipar to

Click to expand...

Thank you, Toppy.

Maylar,


It's been years since I last rebuilt and bled a Norton brake system, but I seem to remember that it was important to lubricate the caliper seals with some brake fluid before assembly.

Also, I once purchased some stainless replacement caliper pistons, and the finish was quite crude. Maybe the pistons are slightly oversized. The leading edges on mine were not finished well, and I had to have some machining done on them in order to fit them without galling the seals.

And when you inserted the pistons into the calipers, how far did you insert them? From your description of the pistons rebounding back, it sounds like they may be installed too far (for the first one, and not far enough for the second). I can't recall for certain if that's even possible, but I have a vague recollection it can be done. I think you'd have brake fluid spilling out if you had, though. Maybe not.

If you can't get it working with fluid, you could remove the master cylinder and try some low pressure air, just to figure out what's going on with the caliper pistons.


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