Bench Bleeding MC's ???

[Mike Braddock]

Greetings from the land of cobra projects that drag on and on

Please excuse my ignorance, but what exactly is this, is it necessary to remove all the air from the hydraulic system, and how is it done?? Thanks !

Mike

[Hotfingrs]

First put the MC in a vise, being careful not to damage it. There is a kit of 2 couplings and 2 pieces of plastic tubing. The couplings screw into where the brakes lines would go. and the tubing hooks to the couplings and goes into the MC resevoirs..then you take something to fit into the bore of the MC where the brake rod would go...phillips driver works well. THen you push slowly back and forth on the MC piston until no more air comes out of the tubing. Then you are done. Of course there will be some air in there once you take the tubing off, then you bleed the brakes at the wheels. This can also be done on the car using the same kit and just pumping the brake pedal.....SLOWLY.....or you'll squirt brake fluid all over the place...

[Roscoe]

Posted by Roscoe:

Bench Bleeding a New or Rebuilt Brake Master Cylinder
by Peter Zaborski, email: peterz at merak.com (replace " at " with @).

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After successfully rebuilding a master cylinder (or when fitting a new one) it is a good idea to bench bleed the MC before installing it in the car. This will fill the MC with fresh air-free fluid and in effect "prime" it for integration with your car's hydraulic brake system, making your on-the-car brake bleeding a little easier.

The basic idea is to create mini hydraulic system on your bench. You can use old brake line fittings if you have them but I didn't so I purchased a master cylinder bleed kit from my local auto parts store. A new MC may include the necessary parts already. The kit should consist of a number of plastic fittings which are designed to fit in the outlets (usually two, front and rear) of your MC. One end of the fittings will be threaded and the other will have a round smooth hose adapter. Thread the appropriate fittings into the outlets on your MC. The kit will also contain a length of plastic hose. My kit had black hose but I found some spare clear hose and used it instead - this will allow viewing of the air bubbles passing through the hose. My kit also had a plastic clip used to hold the two pieces of hose together and clamping to the edge of the fluid reservoir.

Clamp the cylinder firmly in a bench vise so that the top is level. Slide the hoses onto the fittings. Cut the hoses just long enough to reach into the reservoirs and remain submerged - the shorter the length of hose the better. Place the other ends of the hoses into the fluid reservoirs (you'll probably have hold them in place somehow because once you start pumping they'll want to flail around in the air and spray brake fluid everywhere). If you can get a helper that is ideal.

Note: In case anyone has concerns about the recirculation of the fluid, if you don't do this, you will find that the fluid just keeps "falling through" the MC as you do this. I suppose one could try to hold the "waste" container into which the hoses will be inserted at a level where this wouldn't happen but I didn't try that.
Fill the reservoirs with new brake fluid, and pump the piston slowly and evenly, full strokes. I used a big Phillips screwdriver because its tip doesn't damage the piston and the handle gives you something to lean against. I would not worry about the fluid getting recirculated because it is brand new and you are creating a temporary hydraulic circuit with the hoses which will not become contaminated with dirt. The air which is still in the system at this point will be bled out.
Pump the cylinder until the tubing contains no more air bubbles and no new ones emerge from the MC on the down stroke. On my MC this took about 15 strokes some may require more, some less. Keep going until the air stops as this will make the task of bleeding the brakes in the car much simpler.

When all the air is out, mount the cylinder in the car. Here you have to be careful to prevent the fluid still in the hoses from spaying your car and any other painted objects nearby - brake fluid is a great paint remover! If you decide to remove the hoses before installing on the car, make sure to plug up the fittings - I just held the hoses up while transferring from bench to car. Once the MC is mounted in the car, remove the fittings and connect the brake lines. You'll lose a little fluid but the check valves in the cylinder should stop any major leakage.

Now you are ready to bleed the brakes in your car and it should be a lot easier than if this step was avoided.

Peter Zaborski
76 TR6 (CF58310 UO)

[petek]

Bench bleeding has been covered so I'll take a shot at the "is it necessary to eliminate the air from the system".

From what I understand, if you have air in the system the brakes will feel spongy because the air is compressible; think of it taking up more or less room in the brake lines when you step on or release the brake pedal.

[Mike Braddock]

Thanks for the insightful replies! Petek, after rereading my post, I realized I worded that part funny. I didn't mean to ask if it's necessary to remove all the air from the lines, but rather if you must bench bleed the master cylinder in order to be able to remove all the air from the lines. When I bled my clutch master and slave last night, I ran a line from the master cylinder to it's reservoir, filled the reservoir, pumped the pedal about 2 dozen times until I could no longer see air bubbles moving around and then hooked up my slave cylinder in place of this temporary bleed line. Next, I pumped the clutch pedal slowly about 1" stroked until I got a firm pedal and then bled the air out of the slave using the traditional 2 man method. I did this about 10 times and didn't see any more bubbles. The slave cylinder is moving about 3/4", and the pedal effort is surprisingly light. I won't know if I have enough stroke until I fire the engine this week and see if I can disengage the clutch. Thanks again for all the replys. I wasn't sure if I needed to bench bleed it, so when I do my brakes in a few weeks, I'll make it a point to bench bleed those masters as well...

Mike