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Standard vs Power Brake MC -- Follow up.


James_Douglas
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Hi All.

 

My 1949 has a leaking MC. All my spares are at my place out of town. I am using a 1951 Imperial MC and pedal as the power brake cars 1951-1954 used a different pedal and a different push rod. This changed the pedal ratio. Also, the 1951-1954 MC had a 1.50 stroke versus the 1946 to 1950 which had 1.125 stroke.

 

What I noticed on my now that it is on the bench is that when the pedal is in the resting position, up against the stop on the housing, that the face of the plunger is dead even with the face of the MC.

 

My question is: Are the 1946 to 1949 units the same, or does the change in the push rods on the power units from 1951 to 1954 allow the plunger to travel out farther than the non power ones.

 

Since all my hard brake spares out at my place out of town, I cannot check them. So, I wanted to see if anyone has one on the bench and call advise.

 

*************

Well, since I wrote this I got a new master cylinder from one of the usual suspects that claim that this MC is all American made.  I can tell you that the two machine screws that hold the plate and boot to the MC on the new unit were #10 Metric heads. The threads were SAE. Made in the USA, perhaps.

 

I see that the length of the bore for this unit and my 1951 Imperial Unit are the same. All the difference in stroke is in the different pedal ratio and different pushrod.

 

Speaking of that. There is no way to get the pushrod "B" setting to 3/4 inch as is advised in the service manual. The different pedal and pushrod set that geometry and no amount of adjustment can change it. It ends up being about 1.25 to 1.5 inch of total travel before the bleeder port stops bubbling inside the master cylinder. I spent an hour on the bench with it full to set it. I gave it a generous 1/4 inch of free play for the "A" setting. I suspect that nobody at MOPAR had a light bulb go off and amend the service manual.

 

WARNING!

 

The Master Cylinder I got had a pedal pin hole size of .7465 with the pin being .75000 inch. The engineering books tell you that for cast iron housing and steel pin that the press fit should be a maximum of one 1.3 thousands of an inch. My had 3.5 thousands of an inch. So I needed to ream it.  If one tries to press or hammer the pin in with that tight of fit you may crack outright the housing. You may also induce some unseen cracks that could get big and fail in the future.

 

I would suggest that people check that hole if they buy new master cylinders.

 

Best, James

 

 

Edited by James_Douglas
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  • James_Douglas changed the title to Standard vs Power Brake MC -- Follow up.

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