Rear brake master cylinder gone bad?

[Ninja]

After a few months off the road (...and it was just a burst fuel filter!), I was giving the Stelvio some love ready to get back on the road, so i fitted some new brake pads all 'round.

All seemed fine. With the rear, there wasn't much pressure, but I thought a spin around the block and a bit of pumping would sort it out. It was functioning spinning the rear wheel on the centre stand and dabbing the brake pedal with my hand.

But, I was most disappointed to find, there was NO rear brake when I took it round the block for a spin.

Got it back, tried to bleed it, but there wasn't really any fluid making it out of the bleed nipple on the rear caliper. A tiny bit if I really pushed on the brake pedal hard. With the bleep nipple closed, there is NO movement or attempt to move the caliper pistons.

So, I'm guessing the master cylinder has failed/got stuck/leaked or something equally as bad. Maybe the extra travel of the master cylinder 'piston' pushed it where it too far and it's got stuck, or something. I don't know my way around these things, so looking to see what you guys thought, being enormously more knowledgeable than myself.

Oh - plenty of brake fluid in the reservoir. No fluid leaks in the system anywhere (the reservoir level isn't dropping)

I'm discounting a caliper problem as the problem seems to be at the bleed nipple before we get to the caliper.

The bike is an early '08 non-ABS bike. What could possibly go wrong?!

Thanks in advance for any thoughts

 

[scott_m]

Nothing is wrong my friend! Parts have a finite life of their own. Some last longer, others shorter.

If your problem is indeed the pressure piston, then that motorcycle got 9 years service out of that master cylinder and given that we don't know how often the brake fluid was flushed (it should be done annually), that's not a bad run.

I assume that you have attempted to bleed the brake first; Yes? If not, that is step one.

If you did that already, then I would remove the caliper, loosen the brake fluid reservoir cap and physically (gently) push back the pads to retract the piston and make sure that it moves ok (at least in reverse) and you should see the fluid level in the reservoir rise.

If this all checks out, then all that is left to do is you just need to call your local shop and order the rebuild kit. They are not at all hard to rebuild, and then you can refill it and bleed the brakes, and she will be just like she came from the factory.

Good Luck!

 

[Ninja]

Hey Scott,

Yes, indeed, 9yrs is fine in my book too, I've just never had one fail before (and lets just say that is one of my younger bikes I've had in the last 30yrs!).

I was unsure of the symptoms, so here I am!

Yes, tried to bleed (it's all above, so I won't repeat myself too much ), but no fluid coming through.

Pistons in calipers have been pushed back when fitting new pads (which is why I think the master cylinder may have been pushed further along it's travel than it's happy with).

If I need to overhaul the m/cylinder, that's no problem (although when I tried to budge one of the two mounting bolts yesterday it really didn't want to budge, but that problem might be for later )

Thanks

 

[john zibell]

If I need to overhaul the m/cylinder, that's no problem (although when I tried to budge one of the two mounting bolts yesterday it really didn't want to budge, but that problem might be for later )

Thanks

Forget the overhaul of the MC. For not much more than a kit, you get a completely new unit. Also I've given up on rebuilds. The bores of the old MCs are usually so bad they can't be saved. The MC is pretty inexpensive even at list price. https://www.harpermoto.com/rear-brake-pump-05666930.html

 

[canuck1969]

Before you get the rebuild kit or a new MC, try to force some fluid out the caliper. I have had it before where there is enough air in the caliper and line that the MC does not push enough volume to force it out or you have air right at the MC. You need to pull it out.

Take the caliper off the bike and try to get it as low as possible and see if you can get a good stream out of it. If you have a vacuum bleeder to pull the fluid out it will be better. I have used a syringe (like the kind you get for kids medicine) and a piece of tubing and sucked it out. If you can get a good flow, then try a regular bleed.

Worth a try before you spend money on it.

Edit: Reading your post again, are you saying that when you push down on the pedal it is hard and you get no fluid out or is it soft and goes to the bottom of the stroke and no fluid gets pushed out. I think you may have jammed the pistons in the caliper itself. Can happen if the pistons are dirty when you push them back in. Can try a few good wacks of a rubber mallet and pushing the pedal hard and see if it moves. I didn't think you could jamb a piston in a radial MC? Also take the nipple right off and try bleeding again if the mallet does not work. If it is soft, the above still applies.

 

[Ninja]

Forget the overhaul of the MC. For not much more than a kit, you get a completely new unit. Also I've given up on rebuilds. The bores of the old MCs are usually so bad they can't be saved. The MC is pretty inexpensive even at list price. https://www.harpermoto.com/rear-brake-pump-05666930.html

Awesome, thanks John. Not worth me beggaring about for that difference. I'd only muck it up and have to buy a whole new unit anyway

 

[Ninja]

Before you get the rebuild kit or a new MC, try to force some fluid out the caliper. I have had it before where there is enough air in the caliper and line that the MC does not push enough volume to force it out or you have air right at the MC. You need to pull it out.

Take the caliper off the bike and try to get it as low as possible and see if you can get a good stream out of it. If you have a vacuum bleeder to pull the fluid out it will be better. I have used a syringe (like the kind you get for kids medicine) and a piece of tubing and sucked it out. If you can get a good flow, then try a regular bleed.

Worth a try before you spend money on it.

Edit: Reading your post again, are you saying that when you push down on the pedal it is hard and you get no fluid out or is it soft and goes to the bottom of the stroke and no fluid gets pushed out. I think you may have jammed the pistons in the caliper itself. Can happen if the pistons are dirty when you push them back in. Can try a few good wacks of a rubber mallet and pushing the pedal hard and see if it moves. I didn't think you could jamb a piston in a radial MC? Also take the nipple right off and try bleeding again if the mallet does not work. If it is soft, the above still applies.

Yes - the syringe idea is good - I'll wedge one on the end of some clear tubing tomorrow and see what I can suck out.

The pedal is soft - no resistance - and no fluid makes is out of the bleed nipple. No leaks and the reservoir level is not dropping.

I'll try the rubber mallet tomorrow too.

Thanks for the input - gives me something to try!

 

[canuck1969]

Yes - the syringe idea is good - I'll wedge one on the end of some clear tubing tomorrow and see what I can suck out.

The pedal is soft - no resistance - and no fluid makes is out of the bleed nipple. No leaks and the reservoir level is not dropping.

I'll try the rubber mallet tomorrow too.

Thanks for the input - gives me something to try!

If it is soft, just try pulling some fluid through and then try pumping. Don't worry about the mallet. That was if the peddle was hard. It is now either the MC or trapped air that needs to be pulled through.

 

[Ninja]

So, I got a syringe and a bit of pipe on the bleed nipple and tried to pull fluid though, tried to bleed., but no improvement.

Interestingly, I think, if I pushed fluid into the calliper and closed the bleep nipple under pressure, I got some brake. But when I try to bleed again, any pressure disappears.

In other words, fluid is not being replenished via the m/c once it's bled out.

So - a new m/c is on order, but if my conclusion is incorrect and it is repairable or otherwise recoverable, I'll at least have a spare m/c!

 

[john zibell]

So, I got a syringe and a bit of pipe on the bleed nipple and tried to pull fluid though, tried to bleed., but no improvement.

Interestingly, I think, if I pushed fluid into the calliper and closed the bleep nipple under pressure, I got some brake. But when I try to bleed again, any pressure disappears.

In other words, fluid is not being replenished via the m/c once it's bled out.

So - a new m/c is on order, but if my conclusion is incorrect and it is repairable or otherwise recoverable, I'll at least have a spare m/c!

This is curious. This may be an indication the master cylinder is not getting a complete push through. Is the brake pedal obstructed in any way to prevent a complete full push on the MC piston? Is there too much free play in the pedal before MC actuation? The reason I say this is that on some bikes I have encountered this. When I removed the obstruction (usually a muffler but not the case on a Stelvio) a successful bleed was possible.

 

[scott_m]

I've been thinking about this and I remembered that I had seen something very similar to your situation.

The problem turned out the be the foil covering that they put on the openings of brake fluid containers. Some of that had inadvertently fallen into the fluid container, and was subsequently poured into the reservoir on the motorcycle. It sank to the bottom and was caught up against the input hole, thereby preventing build from being allowed to enter the master cylinder.

Initial diagnosis of the mechanic was a failed master cylinder, but during the teardown, the contaminant was found!

Worth a try, but have you evacuated with your syringe, most of the brake fluid from the reservoir and made sure nothing is blocking the intake hole?

 

[Ninja]

This is curious. This may be an indication the master cylinder is not getting a complete push through. Is the brake pedal obstructed in any way to prevent a complete full push on the MC piston? Is there too much free play in the pedal before MC actuation? The reason I say this is that on some bikes I have encountered this. When I removed the obstruction (usually a muffler but not the case on a Stelvio) a successful bleed was possible.

Yeah, lots of travel in pedal - much more than I'm used to seeing in fact! The rod in the MC moves pretty much directly when the pedal moves.

 

[Ninja]

I've been thinking about this and I remembered that I had seen something very similar to your situation.

The problem turned out the be the foil covering that they put on the openings of brake fluid containers. Some of that had inadvertently fallen into the fluid container, and was subsequently poured into the reservoir on the motorcycle. It sank to the bottom and was caught up against the input hole, thereby preventing build from being allowed to enter the master cylinder.

Initial diagnosis of the mechanic was a failed master cylinder, but during the teardown, the contaminant was found!

Worth a try, but have you evacuated with your syringe, most of the brake fluid from the reservoir and made sure nothing is blocking the intake hole?

This is an interesting thought - it's getting dark out now, but I'll go and check in the morning. I'm pretty sure it's clear, but then I haven't actually been looking for anything right at the bottom of the MC, so it's worth checking.

Now, assuming I do go ahead and take the MC off - is it normally a beggar to get the two mounting bolts off? I got a ring spanner on one, but it didn't seem to want to budge. I didn't try particularly hard, but felt pretty solid. I ordered a couple of new bolts along with the MC.

Thanks Guys

 

[john zibell]

If regular bolts, I would recommend using a 6 point socket if you have room to get one on.

 

[scott_m]

Penetrating lubricant spray applied and allowed to soak in overnight is your best friend here!

 

[Ninja]

Well, the new MC arrived this morning, so I didn't waste ay time in getting the old one off (the mounting bolts came off no problem in the end) and the new one on.

Brake worked like a champ squeezey thing in no time

The old one was full of dried, rusty, muddy 'stuff', so not surprised it wasn't working. Plenty of grease applied to new one.

Thanks for all the input and thoughts guys,

 

[shanered6]

Late here but i had the same problem loosing back brake all the time , so i changed the rear master from the 13mm brembo to the 11mm brembo from the Ducati 848 and ive never had a problem since .