hi on my k 100rt 1986 i am having some probs with the rear master cylinder when i try to bleed the brakes the plunger moves in and out a few times then sticks in and wont return out. i have taken it apart cleaned it and all seamed fine then tried again with the same result.would a rebuild kit cure this or could it be the m/cylinder housing needs replacing which would mean new m/cylinder. it has the 13mm one at the moment but i see the the newer one is 12mm if i do need to replace it which one would be best dont want to spend but brakes can be useful or so the wife tells me any help please many thanks from paul uk.

Sorry Paul,

I don't have a lot of experience with master cylinders as after a good cleaning they always worked for me.
My question is: Is that the master cylinder sticking or the pedal?

the pedal is fine its the plunger with the two seals and spring thats sticking works fine when empty but brake fluid in and 5 or 6 pumps with the pedal and sticks in.

In order for your plunger to return it needs to be able to 'pull oil back into the master cylinder'. If you press the plunger in (you brake) the oil is just like a steel rod (to simplify); it only can go around corners, so it pushes the pistons onto the brakepads on the disk. In order for the plunger to return there is a spring on it, but that is a fairly light one. If the pistons in the calipers work freely the spring will be strong enough to have the plunger retract and loosen the pistons. However, when the pistons don't move freely the plunger can't come back as well. Lots of blabla, but I think you need to have a good look at your caliper. Hope it makes sense.

paulcrtny wrote:the pedal is fine its the plunger with the two seals and spring thats sticking works fine when empty but brake fluid in and 5 or 6 pumps with the pedal and sticks in.

I just went through the process of servicing my rear brakes. See my notes on this: http://k100rt.aforumfree.com/tutorial-f17/rear-brembos-service-t1065.htm?highlight=rear+brakes

Essentially it is easy to remove the calliper and it comes apart by removing two bolts. The major thing is to get the two tiny o rings that seal the fluid passages (2 per calliper).

By doing this you can service the hole thing and not waste time by taking the master cylinder apart twice and not improve anything.

ok thanks for all the info thing is just put new calliper with new pads in it all seems fine there both pistons moving freely but have not got any brake fluid to the calliper yet .i have the bleed nipple open with clear tube on and non return valve but then when i pump pedal air starts to come out the end of the valve but when the plunger sticks cant continue and have to take m/cylinder of again to free plunger and now i think i have broken my toes using the kick it as hard as you can method will have to find a hammer

I have found when bleeding the brakes on my 86 K100RT after fitting new master cylinders that the best way to get fluid through the system to the callipers is to suck it through from the bleed nipple on the calliper. I use a vacuum brake bleeder which works on an air line. In the past, before I aquired it I bled the hydraulic clutch on my sister in laws Audi using a large disposable syringe ( 50cc I think ) with a piece of tube attached. My brother had fitted a new master cylinder to it and no amount of pedal pumping or swearing would get fluid to the slave cylinder. Some types of master cylinders seem to airlock when they have been newly installed or emptied and cleaned out and the fluid dosen't flow into them from the resivoir. Therefore the only way to get fluid down into them is to "pull it" by creating a vacuum below or "push it" by applying pressure from above. There may be nothing wrong with your master cylinder. Under normal operation with fluid in the system the piston doesn't travel very far when you press the pedal. With no fluid, or when you are pumping the pedal to try and bleed it the piston can be pushed to the end of the cylinder and can stick there. You can probably get a large disposable syringe quite cheaply at a pharmacy or vets. It is also very handy for filling fork oil but either use a different one or clean it completely.

First off the usual routine with master cylinders is to bench bleed them and then fit them with fluid in them. If you don't do that it is hard to get the air out and fill with fluid completely.
As a tip I find using a MightyVac is a good way of drawing fluid through the system. Don't forget to butter thick silicone grease around the bleed nipple screw thread to stop the air being sucked in there and start a gentle suck through keeping the reservoir topped up. That way you can clear all the air out of the system. Don't forget if you are going to use the pump and release method you have to open and close the bleed nipple in synchronisation with the pumping.
Regards,
K-BIKE