getting a disc brake to pump up

maylar · Sep 13, 2013

I'm helping a friend rebuild the front disc brake on his 72 Commando. We replaced the caliper pistons and seals and the 2 seals and spring in the master cylinder. But we're having problems getting the system to pressurize.

The first problem is that we could not get fluid to flow through the m/c using the lever. Filled the reservoir and no amount of pumping would move fluid. Just a few bubbles out the two little holes in the m/c cup. We removed the lever and pumped the piston with a finger and managed to get fluid to flow, finally to the caliper and out the bleed screw.

After doing this until there were no more bubbles from the bleeder, closing the bleed screw does not restore pressure to the lever. It still goes full travel with no hint of being under pressure.

The m/c is a sleeved Lockheed unit and the caliper is stock (now with stainless pistons). What am I missing?

Thanks

drc

geo46er · Sep 13, 2013

Hi,
Make sure there are no parts of the master cylinder pumping up hill. Turn the bars or even reposition the MC if you have to to get the piston at least in a horizontal position.

GB

Nortiboy · Sep 13, 2013

Make sure the master cylinder piston is free to move to the full open position. ie full range of movement in both directions.

Crack the hose at the master cylinder and bleed at this point if you can. Air collects at the highest point.

Matt Spencer · Sep 13, 2013

Put a neoprene / clear plastic tube on the nipple , And SUCK . Nothing quite like the taste . :x

.

Yea , if theres a ' raised ' bit , its awkward to excavate the air in there . Nipple open 1/4 to 1/3 turn .
can be open ' to far ' . pump QUITELY / slowly . IN & OUT . dont flick . Tube out to can makes life easyer .
A LOT easyer .

B+Bogus · Sep 13, 2013

If it's been sleeved there may be an issue with the position of the sleeve. From what you say, it sounds like the lever can't back up enough to allow oil into the chamber from the reservoir, so the piston is acting like it's too far in. My assumption would be the sleeve isn't far enough into the bore, perhaps?

Which sleeve did you use?

rvich · Sep 13, 2013

If you remove both the brake line and the bleeder from the caliper. Then take a plastic syringe and modify the tip to accept the red straw from a WD-40 can you can use the syringe to inject brake fluid into both of those openings to make sure there is no air trapped in either the front or back cylinders.

Russ

Niagara850 · Sep 13, 2013

Put a resleeved MC on earlier this year. I could pump fluid thru but had little pressure. Set her on the side stand and turned the bars so the MC was at its highest point and left it overnight. The bubbles worked there way up and out and it was perfect the next morning. found the advice in a thread search on the forum.

swooshdave · Sep 13, 2013

Niagara850 said:
Put a resleeved MC on earlier this year. I could pump fluid thru but had little pressure. Set her on the side stand and turned the bars so the MC was at its highest point and left it overnight. The bubbles worked there way up and out and it was perfect the next morning. found the advice in a thread search on the forum.

Clamping the lever back overnight helps too.

rpatton · Sep 13, 2013

You might want to think about getting one of these. It makes fast work of bleeding brakes. None of the opening and closing the bleeder over and over. It works the first time. Painless. $50

http://www.mityvac.com/

swooshdave · Sep 14, 2013

rpatton said:
You might want to think about getting one of these. It makes fast work of bleeding brakes. None of the opening and closing the bleeder over and over. It works the first time. Painless. $50

http://www.mityvac.com/

I have one of those and still need to resort to tricks to get a full bleed. Might be operator error or something.

daveparry · Sep 14, 2013

Although I've never tried it I've read that using a syringe on the bleed nipple and drawing the fluid through that way works well. I remember having awful problems bleeding my 850 after sleeving the m/cylinder, that was about 15 years ago and I've never needed to do it since so have never tried the syringe trick!

Dave.

toppy · Sep 14, 2013

Syringe trick works an there are some on YouTube. I got kit of Ebay less than £10 posted to my door. The kit had two 50ml syringes one is used to take fluid out of reservoir to stop it over flowing as the other pushes fluid up into system through bleed nipple.
Air can be trapped in the inner piston so carefully pump out pistons one at a time as far as you dare then open bleed nipple as you push piston back into its bore this may clear air bubble if you have caliper so bleed nipple is uppermost

bluto · Sep 14, 2013

rpatton said:
You might want to think about getting one of these. It makes fast work of bleeding brakes. None of the opening and closing the bleeder over and over. It works the first time. Painless. $50

http://www.mityvac.com/

+1, sometimes a vacuum bleeder does the trick when all else fails, very handy on cars too when you don't have a helper available....however it can be hard to see when all the air is out of the system as the Mightvac will suck air in past the threads of the bleed nipple, it goes right back out the nipple but aerates the brake fluid in the bleed hose

acotrel · Sep 14, 2013

Take the master cylinder off, and tie it with a piece of string to the rafter over your bike, so that the air can rise straight up into the reservoir. Go and watch TV for the night and bleed it in the morning.

acotrel · Sep 14, 2013

Matt, if you get brake fluid into you, you will probably get a banging headache. I did a test on it once which involved heating, the vapour did the business.

rpatton · Sep 14, 2013

bluto said:
+1, sometimes a vacuum bleeder does the trick when all else fails, very handy on cars too when you don't have a helper available....however it can be hard to see when all the air is out of the system as the Mightvac will suck air in past the threads of the bleed nipple, it goes right back out the nipple but aerates the brake fluid in the bleed hose

I noticed that it will pull air past the threads on the bleeder. I read someplace that you can stop it by putting Teflon tape on the threads. It works, you can still get it to pull air past the bleeder if you get the vacuum high enough but otherwise it works great at lower levels of vacuum. It's important because the presence of bubbles from the bleeder is what tells you that you're not done bleeding the system.

Matt Spencer · Sep 15, 2013

" Matt, if you get brake fluid into you, you will probably get a banging headache "

Id been wondering what caused That ! .

Jed · Sep 15, 2013

The brakes are a bit of a pain but having just got them both working it seems (provided all seals etc are good) then patience is what is needed
Getting the MC higher than the calliper and leaving it is the go
Other than that an old MC cap with a valve fitted and pumped with a bike pump can help

JimC · Sep 15, 2013

Jed said:
Other than that an old MC cap with a valve fitted and pumped with a bike pump can help

That's a form of pressure bleeding. I use a small weed sprayer to hold enough brake fluid to bleed the system and provide pressure. If you are bleeding a system with ABS, pressure bleeding is the way to go.

As for the suction method, I never seem to have much luck. Gravity bleeding seems to work better than suction bleeding. I replaced a stuck caliper on my Dodge Durango in a parts store parking lot and gravity bled. Works fine.

B+Bogus · Sep 15, 2013

I'm still of the opinion that if it 'behaves itself' - to a point - with the lever off, but doesn't with the lever on, it's probably a bit more complicated than just getting gravity to do its stuff more effectively!
Has it just been re-sleeved, or has it previously worked in this configuration?

getting a disc brake to pump up

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