How to Bench Bleed a Break Master cylinder?

[OUTFXD]

The temp has warmed up and the rain has returned to wash the snow away.  So  of to work I go.

 

I bought a new master cylinder, promptly siezed the old Brake pedal pin trying to install it.  Ordered a new brake pedal pin but couldnt get the old one out.  installed all the interior bits in the old master cylinder after removing all the old parts and honing the cylinder sleeve<?> and the Break Pedal Pin hole. 

Installed all the bits from the new master cylinder and installed it in Jacquiline.

 

Now it wont build pressure.  At all.  

 

I checked the technical archives.  I found a thread about installing a master cylinder on a Desoto. It referenced "Bench Bleeding" the master cylinder and gave a quick description of how to do it.  Me being me,  I didnt understand it. 

 

Anyone have a indepth description of the process?  Can it be done on the car or do i  need to remove the master cylinder?  any other tips of advice?

 

Jacquiline,  1946 Plymouth Special Deluxe, 218 flathead 6 with a manual shift three speed.

 

Thanks in advance!

 

[Sniper]

First of all

 

IT IS BRAKE not break.  The brakes stop your car, if they don't you will likely break things when you crash.

 

Sorry, pet peeve, lol.

 

To bleed a master cylinder you are looping the output back to the brake fluid reservoir.  As you actuate the piston inside the master cylinder you are forcing any air out of the master cylinder.  The loop back just directs the output back to the reservoir so you can not make a big mess and reuse the fluid to bleed the master.  Once there are no more bubbles in the fluid coming out of the loop back tube you should be able to connect the master cylinder back up as normal

 

Can you do it in the car?  Yes, but sometimes the fluid likes to squirt up out of the reservoir if you go too fast/hard with the pumping.  Then you need to bleed the rest of the brakes. 

 

 

[OUTFXD]

UPDATE !!! JAN 2024!!! MORON FIXES BRAKES!!!

 

Loosened the  Brake pedal adjustment and pulled the brake pedal up,  Got a stream of bubbles and the level in the master cylinder dropped to about 1/2 way up. 

 

Brakes pumped up good.   Blead the rear brakes, Got a big cough of air out of the rear gunner side.  Pilot side Just blew fluid.  

 

a bit concerned that I have to pull the pedal up to feed fluid to the brake system, and the pedal dosent want to stay  to stay there.

 

Edited January 22 by OUTFXD
Spelling

[nonstop]

Do you have your pedal return spring in place?

[Dan Hiebert]

I just installed a new Raybestos master cylinder in our daughter's Falcon a couple weeks ago.  The instructions specified that it had to be bench bled...or else.  Says the component could be damaged if not bench bled and won't be warranted if you don't.  I've installed several new master cylinders over the years, that was the first one I'd ever bench bled.  No reason for not doing it before that I can remember, probably just a tad lazy.  Never had any problems, although I will note that bleeding the system afterwards (all new brake components in the Falcon) went far easier and quicker than those that I hadn't bench bled. 

 

There are two ways to do it, but I would think either would be really difficult while mounted in our cars, since the master cylinder is under the floorboard.  One is a la Sniper's video with the return lines, the other you plug all but one of those outputs, fill until the fluid runs out the open one, then plug it.  Both methods then require that you slowly pump the cylinder until there are no bubbles, with the plugged method, you have to keep adding fluid, with the return lines, you don't.  You have to secure the cylinder, I used a bench top vise.  Just have to be careful not to damage the unit.  It doesn't take long.  

[OUTFXD]

11 hours ago, nonstop said:

Do you have your pedal return spring in place?

Yup!  Return spring is in place!

[kencombs]

15 hours ago, OUTFXD said:

Yup!  Return spring is in place!

What is preventing the pedal/piston from fully returning?   Spring not strong enough or????

[Los_Control]

On 1/21/2024 at 6:45 PM, OUTFXD said:

bit concerned that I have to pull the pedal up to feed fluid to the break system, and the pedal dosent want to stay  to stay there.

Yeah I think the main issue is, you cant bleed a MC using the correctly adjusted brake pedal/rod. You need to remove it so you can bury the mc piston .... to get all the air out.

Usually just easier on the bench in a vice then on the car.

 

Wonder what you mean by pull the pedal up?

Correctly adjusted brake pedal rod should have some play, maybe 1/4" .... The spring on a brake pedal should pull the pedal back away from the MC.

 

The spring inside the master cylinder will be enough to push the piston all the way back ..... unless the brake pedal rod is stopping it.

 

 

 

[OUTFXD]

9 hours ago, kencombs said:

What is preventing the pedal/piston from fully returning?   Spring not strong enough or????

 

9 hours ago, Los_Control said:

Wonder what you mean by pull the pedal up?

Correctly adjusted brake pedal rod should have some play, maybe 1/4" .... The spring on a brake pedal should pull the pedal back away from the MC.

 

The spring inside the master cylinder will be enough to push the piston all the way back ..... unless the brake pedal rod is stopping it.

I have to admit I am unsure why the brake piston/pedal is not returning fully,  I didnt have this problem before I replaced the master cylinder.