Do you bench bleed the master cylinder?

[79Vette]

This is the 4th 70s/80s GM master cylinder replacement I have done (all on different vehicles, and all work well), and I have never bench bled a MC. Just put it on, connect the brake lines, vacuum bleed at each wheel using a venturi vacuum bleeder, and then take a break for half an hour or so. The pedal goes right to the floor if you try to push it after bleeding the wheel cylinders (obviously, due to air in the MC). But I have found that if you just get all the air out of the lines, proportioning valve and calipers/wheel cylinder and let it sit for a few minutes then the master cylinder will gravity bleed by itself. Top up the fluid after 30-60 min, and the pedal is firm and everything works great.

I've heard other people talk about bench bleeding the master cylinder (and the MC even comes with a little sheet suggesting bench bleeding be performed), but I've never done it personally because it seems like it would make a huge mess trying to connect the brake lines with the MC already filled with fluid. Reading in the 289-20 TM, Section 7-6a covers brake bleeding with a pressure bleeder and makes no mention of bench bleeding the MC. Section 7-6b covers manual bleeding (with an assistant pressing the brake pedal while you open and close bleeder fittings), and requires you to first crack the lines at the MC and depress the pedal to remove air from the MC/fittings.

Vacuum bleeding seems more similar to the manual method in 7-6b than the pressure method in 7-6a, so it seems like bench bleeding the MC would be required, but skipping the bench bleeding/cracking the lines step has always worked for me on everything from Corvettes to CUCVs. I have a method which works for me and I don't plan to change, but I am curious what everyone's preferred method for brake bleeding is. Either pressure or manual bleeding per the TM or vacuum bleeding with a method similar to mine, and if you bench bleed the MC or not.


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[GopherHill]

I do it exactly like you. I never had a problem from not bench bleeding a master cylinder. Anyway I don't like brake fluid drooling on me on installation.

 

[adf5565]

I Just bled my new MC and brake system yesterday actually. Did it by manually pressure bleeding, went with half pumps of the peddle at first just to get some fluid in the system and eventually worked my way to normal. Didn’t bench bleed the MC and didn’t have any problems

 

[Skinny]

I've worked in cars for 23 years and never bench bled. Vacuum bleeding solves a lot of problems.

In my storage unit I have no air so I use a check valve bleeder in a small container filled with fluid. Pumpy Pumpy and it pushes the air out. Whatever it pulls back in it's pulling from a container with fluid in it.

Never had any issues with either procedure.

 

[Mullaney]

I have done the MC with and without pre-bleeding. The BIG plus with the pre-bleed is that once is enough - BUT - I spent the time to find a pair of inverted flair PLUGS. They have been in my toolbox drawer for YEARS and I honestly don't know where they came from... But, pre-bleeding then sticking in the plugs to keep the mess contained would be save a lot of grumbling about "brake fluid drool".