is it possible to bench bleed a master cylinder by capping the output lines off and pumping it?

i didnt have much luck running the lines back into the reservoir and pumping it so i reverted to the above method and it seems to have worked on two master cylinders.

 

yes, thats exactly what bench bleeding is. and they suggest it before installing

 

i just never heard mention of bleeding it by capping it off. just running the lines back into the mc or back flowing it. i actually set out to test a mc by capping it and just thought to try to bleed one that way. it seems a much easier way to go about it.

 

ya, if you check the instructions when you get a new master cylinder, it says to use the supplied caps and bench bleed it. and yes, much much faster when you get it in the truck. back flowing is the best way to get any and all air out, just the kits are kinda expensive. theres also a pressure system that goes on the master cylinder, you fill the tank with fluid, pump it to pressurize it and then just crack the brake bleed fittings and it will push out. also, costs a few bucks. $50 or so iirc

 

how are you going to push air out of it though if you don't run lines back into itself?
i don't see how anything is accomplished by simply blocking off the ports and pumping.you'll remain where you started lol.
i think you must have just bled the systems through all the lines (which of course works too eventually) and thought it worked.

 

no, i never connect the lines to the mc. i just capped them off and watched the air come out via the reservoir ports. brake pedal barely moves so i guess that it did work. worked twice. once on the bench and once in the vehicle. didnt even have the reservoir on the one i did on the bench.

ive never done it on a car but do it all the time on bikes. just lock the brakes in the applied position and any trapped air rises to the top overnight. same principal i guess.

i never hooked the lines back up as i think i have a faulty ride height sensor which i need to get replace before ill bother.

 

the air pushes back though the master cylinder reservoir

 

this is from the "cardone" brake parts web site, but same thing


The most common problem customers face when installing master cylinders is improper bleeding procedures. In the past, CARDONE supplied a bleeder kit that consisted of plugs with holes. Tubes pressed on to the holes in order to bench bleed the master cylinder. Although this kit was successful when used properly, it was not the best process. One of our goals in Technical Services is to improve every process. We found a better way.

The new bleeder kit comes with solid plugs to bench bleed the replacement unit.

The advantages of the new process are:

More efficient to bleed the master cylinder.

No mess or fluid dripping when bench bleeding or when installing the unit on the vehicle.

Proving the unit is functional during bench bleeding. If the piston is firm when bench bleeding and not on the vehicle when bleeding, the brake system is at fault, NOT THE MASTER CYLINDER.

The following process is supplied with each CARDONE replacement master cylinder. You can help our customers by stressing the importance of following every process we include with our parts, to prevent warranties and increase sales.

1) Transfer proportioning valves, switches or other components to the replacement master cylinder (as applicable).

2) Tighten bleeder plugs into ports (bleeder plugs may be in parts kit or master cylinder reservoir).

3) Support the master cylinder in a vise in a level position. Never clamp onto the body of the master cylinder.

NOTE: Units with remote reservoirs should be bled on the vehicle. It is VERY IMPORTANT that the body of the master cylinder be kept level during the bleeding process.

4) Fill reservoirs halfway or until reservoir ports are covered. Always use new brake fluid from a sealed container as specified by the vehicle manufacturer.

5) Install reservoir lid so brake fluid does not spray from reservoir during bleeding process.

6) Using a blunt tool or dowel (brake pedal if mounted in vehicle), SLOWLY press in and release the master cylinder piston using short strokes of 3/4” to 1”. NEVER STROKE PISTON MORE THAN 1”. Repeat this step until resistance to piston movement is firm and less than 1/8”.

IMPORTANT NOTE: On step-bore master cylinders, WAIT 20 SECONDS BETWEEN STROKES. This will allow air trapped in the quick take-up valve to rise into reservoir. MASTER CYLINDER BODY MUST BE LEVEL.

7) Install master cylinder on vehicle. DO NOT remove bleeder plugs until brake lines are ready to be installed. This prevents air from entering the unit and brake fluid leaking out.

8) Remove one bleeder plug at a time from the master cylinder and connect the proper brake line to that port. Repeat this step for remaining brake line ports. Be sure fluid level is maintained during installation of brake lines and that all fittings are clean before installation. Bleed brake system according to vehicle manufacturer’s procedures and specifications.

9) Verify proper pedal action before moving vehicle. Do not road test vehicle until correct pedal feel and travel are obtained.

 

cool!