Forks done, front caliper done, now I need to rebuild the MC

[jknappsax]

I finally got the Progressive fork springs for my 1979 GS 1000, put the front end together, and then tried to bleed my front brake. I just kept getting air bubbles, finally when I thought it was completely bled, I refilled the master cylinder, and gently pumped . Usually the pressure comes up, and the lever feels firm. No go. There are no leaks anywhere, so I'm figuring a mc rebuild. Any opinions? I thought that I'd finally done enough for the bike to be ridden, but it's always something!

[azr]

I once had a similar problem, just couldn't get pressure to build up. Had to fill the calipers manually to overcome it, she finally started to build pressure after that. I 'think' this can happen sometimes when the system has been drained. I'm sure someone with more knowledge on the issue will chime in though.

[Nessism]

The problem is the master cylinder is not primed with fluid. The port to feed fluid down to the piston is very small and easily plugged with gunk and/or an air bubble. You can sometimes overcome this issue by applying a vacuum to one of the caliper bleeder nipples, thus pulling fluid out of the reservoir and into the pumping chamber. Before doing this though it's best to tear down the master and make sure it's really clean, including that small feed passage port, and also make sure the aluminum in the master is not pitted (which is quite common). Assuming all is clean, reassemble and then do the priming as described and it should start pumping.

[dardoonk]

This Sucks

Vacuum Bleed Kit from Harbor Freight. $25 bucks... Maybe other's less expensive, but I don't ever comparison shop against HF.

[Octain]

I've found the brake system is much easier to build pressure with a newly rebuilt master cylinder.
Just fill the reservoir and pump, and keep it topped off while doing so.
No special procedures or tools needed.
I do like to remove the bleeder nipple and fill the calipers with fluid using a syringe but I dont think this is necessary, just makes the process faster.

A rebuild kit is around 25-30 bux for most bikes. I've use OEM and aftermarket and cant tell a difference.

[Nessism]

A rebuild kit is around 25-30 bux for most bikes. I've use OEM and aftermarket and cant tell a difference.

I agree with rebuilding and replacing the piston/cup/seals. My experience with aftermarket parts, specifically K&L parts, has been very poor though. One master cylinder piston was too large and scraped inside the cylinder bore, and a set of caliper piston seals were too tight, thus they didn't allow the piston to move out to meet the disc properly (hand lever was very soft). In both instances the bike was repaired easily with OE brake system parts.

[cowboyup3371]

Like Nessism said, make sure the return port is clear. Heck, make sure it is even there in the first place. Mine wasn't drilled for some reason and I had to find a shop to do it.

[7981GS]

I once had a similar problem, just couldn't get pressure to build up. Had to fill the calipers manually to overcome it, she finally started to build pressure after that. I 'think' this can happen sometimes when the system has been drained. I'm sure someone with more knowledge on the issue will chime in though.

After plenty of and getting upset that it was taking FOREVER, I did this:



I drew fluid in from the glass and from the master cylinder.

Apply M/C and hold, release bleeder. Wait for air to escape and close bleeder.
Repeat until the fluid was solid, with no air, all the way to the glass.
Then, apply M/C and wait for all of the air to make it into the loop into the glass.
Then, release M/C and the fluid filled FROM the glass.
Complicated but it worked!

Edit: I had 4 bleeders to deal with though.

Daniel

[1_v8_merc]

Ughh I'm having the same trouble with an 82 cb 900ss.

The fronts were Locked...then I got them Unlocked and pistons to back out, but now they won't ****ing squeeze In!!!

I think the master is screwed....


Plus the rear Pistons are corroded shut!
I had to bang the caliper off with a plastic mallet JUST to get the caliper off the rotor!

[Nessism]

Ughh I'm having the same trouble with an 82 cb 900ss.

The fronts were Locked...then I got them Unlocked and pistons to back out, but now they won't ****ing squeeze In!!!

I think the master is screwed....


Plus the rear Pistons are corroded shut!
I had to bang the caliper off with a plastic mallet JUST to get the caliper off the rotor!

Geeze man, how many clues do you need before you will tear those suckers down? Please don't do something stupid like try to ride that bike and go and get yourself killed.

[jknappsax]

It looks like I'm taking the front mc apart; it doesn't seem too hard (Ha Ha Ha!), and a visual inspection is the best way to figure this out. Thing is, I've rebuilt master cylinders, and bled a system after it had been completely drained, but this is the first time I've ever had a problem like this. And Merc, I agree with nessism; I think it is a caliper problem, but it could be a mc. In any case, best to pull stuff apart to make sure.The HF system for $25 looks to be an alternative the the $250 to $300 Mity vac system.