I'm about to install a new mc. It's a single pot under floor unit. I have speed bleeders and Mity vac if necessary. I'm doing the job solo. Do I still need to bench bleed? Posted from the TJJ App for iPhone & iPad
Ok - just so I have the procedure correct - basically run a line from the outlet back into the reservoir and pump until no bubbles, right? Sorry - it's been a while! Posted from the TJJ App for iPhone & iPad
You bench bleed because sometimes the the pedal will not compress the master all the way to the bottom and leave air in the master at the very end and give a soft pedal.
You're right on the procedure. Clamp it in a vise, then put return lines into the wells and slowly depress with a plunger until you don't get bubbles. Too fast and you'll continually get bubbles, so go slowly.
I let them drip without a hose until they are running steady. then I install the hose back up to the reservoir and then pump. I just like to do it the messy way.
Single piston brake and clutch master cylinders should not be bench blead. Potential overstroking (bottoming out the piston) could occur which may damage the internal seal or components.
I had so much trouble bench bleeding one of my projects I dug out an old brake manual from the '60's and it pointed out if air bubbles keep appearing in the MC you have to hold your fingers over the ends of the plastic lines in the fluid at the end of the stroke so as not to pull air back into the bleed lines. I did it and it worked. Just a note of info.
Some of the M/C now just come with a plug that blocks off the line port and you pump till the plunger wont move any more dont ask me how it works but it does.
You got it right on with the procedure. Run small brake lines from the reservoir back into the MC reservoir that is filled with brake fluid. I use a large Phillips screw driver acting as the brake pedal to slowly bleed the system. It is bled once you can do this without any air bubbles.
I've in a pinch, just covered the holes with my finger and it worked fine....but I would only do that in a emergency.
Just to clarify putting fingers over the end of the hoses, that is to prevent sucking in air bubbles that might have been created in the process and reintroducing air into the system which is apparently what was happening to me
Simply not true. The master cylinder piston(s) should be full stroked to purge all the air. The pistons also have to be able to fully bottom out before the pedal hits the floor or firewall.
i bought a master cylinder and in the instructions it said to NOT bottom the piston while bench bleeding.
there has to be a gap between the pedal to the floor and the rod bottoming the piston. Once the fluid is in it's not even close. You can bleed the MC on the car with a screwdriver and and then bleed all the way to the wheels cylinders. Just a lot more air in the lines than needs to be.
Interesting. Bob, every publication I have ever read regarding hydraulic master cylinders has said just the opposite, that the pedal should have a positive stop before bottoming the piston in the bore. In fact, with every new or re-manufactured master I have received with instructions on bleeding state that you should be careful NOT to bottom the piston while bench bleeding. I would even go so far as to say that if the master cylinder is used that pushing the piston and cup farther than standard operating range usually ruins the cup because of debris/rust/contaminates built up past that range. (been my experience) GOOGLE SEARCH ON THE SUBJECT
Can you list any of the publications you refer to? All the new or rebuilt masters I've bought state to fully stroke during bench bleeding, but even if they didn't, I would, as there is no good reason not to. I worked for (and retired from) a very well known company that manufactured master cylinders, and there was never any talk or warning, written or otherwise, about "over stroking". I've never seen or heard of brake pedal positive stops, as they don't exist on any vehicles I've ever worked on, know about or owned, but if they were necessary, how would they be adjusted? This would require precise measuring equipment and procedure, as there is only about an 1 1/4" total push rod travel on most masters. Limiting the piston travel would require a longer cylinder casting, to still allow the necessary fluid volume needed for the calipers and/or wheel cylinders. This would add needless waste and expense. The pistons and cups are designed to be fully bottomed without any damage. One of the first things to check before any dynamic vehicle brake testing is to make sure the pedal bottoms the master before running out of travel, and this sometimes requires maximum pedal force of 150-200 lbs while both primary and secondary circuits are open. I've never damaged the 100s of masters I've performed this on. Full stroking while flushing the fluid every 5 years or so will help prevent any build up of foreign matter deep in the bore that could damage the cups. PS-Congratulations on the great magazine coverage of your very nice chassis parts!
I agree. I was having one heck of a time with my '48 Dodge truck, thought I did something wrong when I rebuilt the M/C. Bought a new one, bench bled, now the thing is TIGHT. Viva La HAMB
This stroking crap again. Manufacturers packaged instructions with the new master cylinder say this in print so big and bold that Stevie Wonder could almost see it.Do Not bottom out the plunger while bench bleeding. Doing so will void the warranty You all can stroke your junk however you see fit or whatever you think is right. I don't care how you stroke it , but I'd read the instructions from who ever is selling it & not the internet.
Also, on the technique, the m/c should be pointing downwards ever so slightly (away from the input end) such that a pocket of air is not able to form at the end. Chris
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