i have a leaking rear master cylinder so i decided to take out the piston to inspect the seals and clean the bore. fairly easy job but i didn't flush the system per the clymer manual. i pinched the reservoir hose, took it out, cleaned everything and put it back together. i pushed the piston in & out several times to expel any bubbles from the reservoir but there was none. put everything back together but when i pressed the foot brake the piston got stuck inside the bore and of course the rear brake was locked. attched a hose in plastic bottle to the rear caliper bleeder, loosened it and rear wheel was free. but piston still won't come out. how do i get it out at this point???? thanks.

A nozzle backed by a healthy shot of compressed air down the opening at the other end does wonders...pointed away from any people or objects...unless you're not fond of them.

You did slide the piston back in right end round?

thanks, got it out by wedging a bolt wrapped in duct tape in the piston shaft opening, pulled down on bolt while pulling out. and yes i had it in correctly but i probably didn't put enough brake fluid in the bore and the piston before inserting it. turned an easy job into a project but i now have fresh fluid in the system w/ no more leaks!!!

thought i had problem fixed but celebrated too soon. rear master cylinder was leaking so i took it apart & cleaned it while still on bike. did that w/ front mc a few weeks ago and had no problem. after i reassembled it i just pushed the piston in & out a few times to purge bubbles in reservoir and it was good to go. thought i could do the same w/ rear but not happening. i've been bleeding it all day w/ mighty vac and still have a spongy pedal even though no bubbles in fluid. caliper pistons are not binding & move freely. before i remove the mc and bench bleed it i was wondering if brake pads are too low would that cause the piston to travel too far and get stuck???? although before i took it apart it was fine other than leaking. in the meantime i've taken it apart so many times in the past 2 days that i stripped one of the bolts holding down the mc. not sure if i frigged the threads of the bolt or the opening but i'll have to address that later. any ideas on both issues? thanks.

Topics merged...member needs some advice and help still. Takers?

Let's try to understand how the brake works and see what doesn't with yours.

Others will chip in (please?) if I make an error, but here it goes.

Brake system is completely filled with oil (non-compressible; means volume is constant. Air/gas can be compressed and therefore change volume).
There is an open connection within the whole system; calliper area behind the pistons, lines, master cylinder and reservoir. The MC and reservoir are connected through the bleed hole in the MC. This bleed hole gets closed of when the piston of the MC is pressed in when you brake. With the constant volume this means the oil is pressed out of the MC and will push the pistons in the callipers out. Those will clamp the brake disk and brakes the bike.
When you release the brake the spring will push/pull the piston back, pulling the oil from behind the calliper pistons and 'sucking' them back into the caliper. Once the MC piston passes the bleed hole you have an open system again and the pressure equalises.

The front brakes are well laid out (relatively ) as everything is in a vertical line up and air will accumulate up at the MC/reservoir (sometimes with a little coaching like keeping the pressure on for some time. This will reduce the size of the air bubbles (you compress them) and allow them to easier travel through the system and release themselves if 'stuck' in a place). With the rear brake the arrangement is different, as the calliper is above the MC and the reservoir is removed from the MC. Normally you would expect air to accumulate in the callipers and/or bleed of into the reservoir if in that line.

In order for the piston to easily push the volume to the callipers you need the reservoir to easily provide oil, also behind the piston (remember it is a closed system). This means that the system should be able to draw in air, which it does at the caps of the reservoirs. In order to prevent humid air to get in contact with the oil you have seals fitted, but you need to be able to get air behind the rubber. Therefore the forward reservoir has air channels in the cap (it also has two bleed holes to the MC), the rear reservoir has some arrangement in the rubber sealing, where the cap sits IIRC.

There have been reports of brake lines being faulty, where using the brakes ended up with the oil in the brake line lining, instead of putting full pressure on the calliper pistons, or where a porous line between the rear MC allowed air to come into the system (possibly combined with an unability to get air in the reservoir top). Other issues were with the lining of the brake lines coming undone and preventing free oil movement.

I have seen a lot of issues getting the rear brake properly bled and normally take the calliper off and put it on the ground with the calliper pistons pressed in, using the method of keeping it under pressure overnight.

You need to make sure that with the MC piston in its 'rest' position (no pressure applied) the circuit is open. (you put the little screw back in with the piston in compressed position, right?)

If you can not get it right with the normal methods (and I think you've done that) I would have a hard look at how old the lines are and renew them and see if that works.

Hope the above helps a bit to focus on a solution. If not hopefully I haven't confused you further

Good luck!

hi rene , thanks for the detailed explanation but everything was fine until i took the mc apart. i thought maybe i needed to take it off and bench bleed it but i'm getting fluid to the caliper and breal pressure, just not enought. i think my problem is tiny air bubbles. the mighty vac instructions says it's ok to see tiny bubbles but i read a post here where someone had issues with the tiny bubbles and had to apply silicone grease to the threads of the bleeder valve. if i understood it correctly, it appears that i need to put a nice bead on the base of the nipple. i can't imgaine i need to take the bleeder valve off to do thisd although i've read about some use teflon tape on the treads to eliminate any air leaks but i would thing it would be eaten away by the brake fluid. so at this point i need advice on how to seal the bleeder valve from sucking in air while i bleed the system w/ the mighty vac. mine is a '91 k100rs 4v w/ abs-I so iread i need to bleed thh modulator first and then the caliper. i didn't do that so last night i bled the modulator was totally free of bubbles. it's the claziper bleeder valve that's showing tiny air bubles that nver go away. hope this clarifies my dilemna. thanks.

If you are concerned that air is coming in through the threads of the bleed valve I would indeed use teflon tape, but make sure you don't get it on the taper at the tip. This is where it closes off after bleeding once you tighten it.

I used a piece if brake line and screwed it in in place of the bleed nipple, that way you get a good seal then when finished replace the nippleand do a final bleed by hand just in case a bubble got in at change over (I have never had this happen).