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Brake Master Cylinder Freeplay

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Alan W.

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I will be replacing the brake master cylinder on my bn6 and can not find any information on setting the push-rod free play. There is no adjustment on the rod and while the service manual mentions free play, it does not describe how to adjust.
Attached is a photo of the existing MC pushrod under the dash. There is no length adjustment. And the photo of the replacement MC shows two pushrods, one with no adjustment, and the threaded one which is not applicable for my setup installing in a 1959 bn6.
How is the freeplay adjusted?
Alan
 

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Hi Allen,

There is no stop in the pedal assembly, so the spring lifts the pedal until the push rod is pulled out against the snap ring that keeps it in the master cylinder. Free play is the movement allowed between the snap ring and the piston and not adjustable.

There are shims (packing pieces - Moss part # 021-488) that can be placed under the master cylinder mounting flanges to adjust pedal height so they are both the same - your car may or may not already have some under either the brake or clutch or both.

Dave
 
I keep reading about pedal height. Some clutch cars, the brake and clutch are designed to be a different height, not a big deal. I certainly wouldn't shim the MC to change the pedal height. The shim is maybe 1/64 of an inch. The shim is designed to make sure the slack is correct.
 
kodpkd said:
I keep reading about pedal height. Some clutch cars, the brake and clutch are designed to be a different height, not a big deal. I certainly wouldn't shim the MC to change the pedal height. The shim is maybe 1/64 of an inch. The shim is designed to make sure the slack is correct.
Click to expand...
While some cars may well have been built with differing pedal heights, that is/was not the case with Austin Healey's. If you read my post above, the stop for the pedal travel is inside the cylinder itself, not in the pedal assembly. The pedal itself is free to swing to any height it wants and is only limited by the internal stop in the master cylinder - there is a snap ring keeping a washer from coming out of the cylinder and any "free play" is between the retaining washer and the piston therefore, no adjustment for 'slack'.

Originally these cars had fixed length pushrods (some have installed aftermarket pushrods with threaded clevises that are adjustable, but they are not OEM). Hense the "shims" under the master cylinders to adjust pedal height.

The shims/packing pieces are not 1/64 (.015). I have several I've collected over the years from several cars, the ones I have average .065 thick.

The ratio between the pedal swing and the pushrod swing is about 4:1 for the brakes and about 4.63:1 for the clutch. That means the clutch pedal height is altered by .300" with one shim and the brake pedal height is altered by .260" with one shim.

3 pictures attached - 1st is pedal assembly removed from the car showing examples of possible pedal swings, 2nd is shims in question, 3rd is sketch of pedal showing relationship between the pedal and the pushrod clevis attachment points.
 

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OK, I exaggerated the thickness of the shim, my bad. I still wouldn't use shims to adjust the pedal height. What difference does it make if one pedal is different than the other. My other car, there is about 1/2 inch difference and it doesn't make any difference in how it drives. In fact I would be more concerned about the difference in the throttle and the brake, not the brake and clutch.
 
My response was inspired by your statement "The shim is designed to make sure the slack is correct." which is wrong because the shim would have no effect on slack/clearance since there is no stop in the pedal assembly. Why do you suppose they had shims? As to whether you want to use shims or not, your car do what you want.
 
If you move the MC forward you will keep the slack in the rod to MC and the slack in the rod to pedal. If you can move the MC backwards you are pushing against the pedal and the MC end of the rod, there won't be any slack. As soon as you start moving the pedal you will be pushing against the MC piston. No slack.
 
I'm probably not explaining myself very well, but I'll give it one more try.... The only slack present is from the clearances between the clevis pin, the holes in the clevis, and the clevis pin hole in the pedal. This clearance (slack) is very minor when new. With age the clevis pin, clevis, or clevis hole can wear causing more and more slack - but it can't be adjusted out, you have to replace worn parts to correct excessive slack caused by wear.

The return spiring lifts the pedal until the washer in the master hits the snap ring at the end of the cylinder. When you press on the pedal, you take up this small clevis pin slack before the piston moves. Moving the MC forwards or rearwards will not change the amount of slack, just the pedal height.
 
My assumption was that there is a stop in the pedal as it returns. That isn't true. You are correct, I am wrong. Sorry. I don't know how far the pedal will spring back when not connected to the MC through the rod.
Just a thought, maybe that would be a good thing to build into the pedal. Then it could be adjusted to not have any slack as you start pushing the pedal.
 
The only slack will be it the clevis pin, clevis or pedal mount bearing. If there is too much, replacement of the pin or pushrod is the only solution if using stock parts.
 
Yes, I did install a new rod and pin for the brakes and a complete MC for the clutch. That helped a lot.
 
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