Master cylinder rebuild

[Marti1947]

I just had the master cylinder on my 1947 Dodge 1/2 ton bead blasted and honed. When looking  in the resivoir there is a small hole and a larger hole going into the cylinder. The larger hole is drilled through into the cylinder but the smaller one does not go through to the cylinder channel. I am reluctant to drill it out. Can it function with one through hole or do you think the honing made it seal up. When I feel in the cylinder wall it is smooth and I do not see that it ever was all the way through from the resevior.

[Los_Control]

Seems like I also looked at that hole and thought the same thing .... mine was not all the way through and I think it has a different purpose.

I believe it is exactly how it should be and I would do nothing to it .... unless it does not work.

 

You can only hone them so far before they become too large to be safe.

What I mean is, I rebuilt mine and bled them out and the brakes felt good .... then I pretended to do a panic stop and slammed on the pedal as hard as I could.

The pedal went straight to the floor ... worked fine in a normal situation, but if needed to stomp on them in a emergency they did not work.

 

I only say this because of a aggressive cleaning with sandblasting then honed smooth ... has it ever been honed in the past from a previous owner?

Go ahead and put your kit in it and bleed the brakes and do try the panic stop .... either it works or it does not .... do not drill the other hole through.

 

If it does not work, you have options to buy a new China replacement, have your drilled and sleeved and professionally rebuilt ... or go with a modern conversion.

 

When I was a 17 year old kid I worked in a Texaco gas station and I rebuilt brakes for the mechanics. I would hone the wheel cylinders out and then install a rubber cup one size larger then what I took out .... they either worked or they failed the panic stop test.

I was told they could be rebuilt safely 1 time, then toss and replace with new .... this was in the 70's and how brakes were dealt with.

Was a rule of thumb and same with the master cylinder.

 

Make sure you clean it out properly and throw it together and try it out ... just do not get lost in the weeds trying to save something that is too far gone.

Having one sleeved is not out of the question.

[Harley PHD]

The small hole should be a very small hole.  The brakes wont bleed with it clogged up.  I've had to use a sewing needle to clean them out, be careful, you might have to hone it a little if you nick it up with the needle.

[Roofus]

If the small hole is blocked, there will remain a residual pressure in the brake cylinders, the toppings will stay in contact with the drums and they will overheat.

It is therefore vital that it is open, it is quite easy to check, when you press the brake pedal, a small geyser must spring into the reservoir at the start of the race.

Edited Friday at 07:23 AM by Roofus

[soth122003]

The small hole is to relieve the line pressure after the brakes are released. The 2nd rubber cup on the MC piston sits between the large and small holes when the brake is at rest. When you do a pedal adjustment, make sure the cup does not cover the hole or the brakes build up pressure and will not release. It must be open. Best thing to use is a piece of .020" safety wire. A bread twist tie can work as well. I usually put the wire in the hole when I make pedal adjustments to make sure the cup doesn't cover the hole, but comes right up to it. 

 

Joe Lee