Thread: Bench bleeding new master brake cylinder question.

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Hello fellow Club HotRod members,

I am fairly new member and need some advice. I have just completed the hydraulic brakes system on my 37 Ford coupe project. Discs in front and drums in the rear. Every component in the system (master cylinder lines, residual valves, proportioning valve, tees, flex hoses, and slave cylinders are new and dry). No fluid has yet been put into any of the system. My question is this, do I need to do a bench bleed on the new master cylinder disconnected from the system since all components are dry and full of air now? Any help would be greatly appreciated. Thanks! Mickeystan

First, welcome to CHR even though you've been around a while! That's a good question, and I look forward to the answers from others who've done this more than me. Maybe I should have, but when I did mine I didn't bench bleed the MC first, and in fact did not even think about taking it off the car to bench bleed. My MC is frame mounted, and I just filled it up and hand pumped the brake, adding fluid to keep it full. Once it quit dropping I loosened the RR bleeder, put a piece of small ID rubber hose over the end of the bleeder, hanging into a clear bottle with enough fluid in the bottle to cover the end of the hose. Hand pumped until that one quit blowing bubbles, and moved to LR, then to RF and ended with LF. May not have been right, but it worked for me.

Yes, always bench bleed.

If yo don't get all the air out on the bench, it will become very obvious while bleeding the servos, as you wont achieve a high, hard pedal.

A tip if you find yourself in this position: you can get the remaining air out of the master as thus:

With the system full, and most air removed, press the pedal slowly to the floor. Release quickly. Repeat. The remaining air will bubble out of the master through the reservoir.

I always bench bleed. They used to include a kit when you bought the MC but I haven't gotten one lately.. You need two fittings for the MC which I might add are not standard to anything, and seems all manufactures use different size fittings. So when changing the MC you have to be carful not to destroy the nut.. You can put new tubing in the nut but finding a new nut is a pain. I bench bleed, then do not remove the 2 lines from the fittings to the top untill the MC n is mounted. then quickly put the brake lines on letting a little fluid excape. Then depending on what you changed it for you can go bleed the system or just check to be sure you have brakes. Boy that long winded. Its easier to do than tell how

I never bench bleed especially if I've replaced lines or calipers / wheel cylinder(s). I prefer to use that opportunity to flush any contaminants out. When replacing just a master, I never recycle what was pushed through back into the reservoir. Old assembly lube, bit of dust / dirt whatever.. just as soon get it out of the system.

I pretty much follow Rspears routine.

Thanks guys for your responses as it is helpful to see what others have done or would do. The reason I am asking is it seems to me it cannot hurt to do the initial bleeding of the new master cylinder into the system since everything is new and has no fluid in it anywhere. There is only air currently and what can a few more air bubbles do since they will get pushed out anyway? I know there is a small chance that some will be difficult to remove but the system is chuck full of air now so I am wondering if a little more air is really harmful. Obviously I will have to repeatedly pump the brake pedal and keep and eye on the fluid levels in the reservoir until all air is out of the system as I am purging each of the wheel slave cylinders. If others have input to make I would appreciate there expertise and experience. I am still a few days away of adding brake fluid to the system. Again, thanks to all who have shard their recommendations and experience! Mickeystan

OH... and WELCOME Mickeystan!!

I made a fluid vacuum assist from an old mason jar. I soldered two pieces of tubing into the lid. One is for a tubing piece which attaches to the blender fitting at the servo. The other goes to a vacuum pump. Then, I turn on the pump, and sit at the master adding fluid. The mason jar becomes a fluid reservoir, catching the fluid and air. It draws air around the fitting though.

I did almost the same. Made an adapter to pressurize a gallon can with air on top, a tube just above the bottom would carry the pressurized fluid through a hose to another flat adapter clamped to the top of the master. A thick rubber sheet glued to the flat plate kept the fluid in. Then I could just pop open the bleeders one at a time.

I agree 100% that bench bleeding is mandatory anytime you're working on an existing system, but for a first time fill, no fluid anywhere (all lines, all wheel cylinders/calipers empty) I don't see bench bleeding as adding any value. The whole system is filled with air, and bench bleeding is not really going to save much time, if any, IMO.