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Incorrect Master Cylinder ?

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farna View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: Dec/10/2021 at 7:13am
Some of the Wagner master cylinders were stepped, but AFAIK all the Bendix are the same bore for front and rear. The original brake lines for the Wagners usually had different sized flare nuts for front and rear so they couldn't be mixed (line diameter was the same, just nut size was different). Bendix usually use the same size nut unless the line sizes are different. Line sizes are usually the same for drum brakes, sometime front discs use a larger line for front than back. Easy to change the lines --- same size nuts on the other end (junction block).
Frank Swygert
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Stingray Quote  Post ReplyReply Direct Link To This Post Posted: Dec/10/2021 at 10:51pm
Thanks Tomj and Frank,

I've had all the drums off and shoes adjusted OK and no wheel cylinder leaks.  (Front drums are a PIA as the hub has to come off the spindle with the drum.) 

My understanding is the hydraulic pressure will be the same front and rear (with a good m/c) but volume displaced will be higher with larger bore.  I don't know whether my m/c is stepped or 1" front and rear. 

My next steps are ...
Bench bleed master cylinder
Fit brake line extenders I've made to swap front and rear lines
Bleed brakes and test drive.

If problem goes away, might have been insufficient volume to front cylinders (unlikely?) or defective m/c and I've just moved the bad circuit to the rear, or bench bleeding fixed it. Confused

If the problem is unchanged, I still have a front brake problem but it is not the m/c.

I'll keep you posted...might be a week before I get to finish the work.

Beer
Joe




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Post Options Post Options   Thanks (0) Thanks(0)   Quote tomj Quote  Post ReplyReply Direct Link To This Post Posted: Dec/11/2021 at 1:50am
THe pressure works as follows:

* for a given master cylinder size, a larger wheel cylinder gets more pressure. Smaller wheel cyl, lower pressure.

If the master cyl piston is 1 square inch (it's not, lol) and you put 100 lbs of force on the piston, that's 100 pounds/square inch. 100 psi on a piston that's 1.2 sq in area will produce 120 lbs of force.

If you put a 2 sq in wheel cyl in you get 200 lbs of pressure -- twice the force. The catch is, that much fluid volume requires the master to move twice as far. So too small, you run out of stroke.

Maximum stroke on ordinary cast-iron masters is about one inch -- an educated guess from engineers at C H Topping when I asked them when I was designing the brakes in my roadster. Actual stroke is about half that.


1960 Rambler Super two-door wagon, OHV auto
1961 Roadster American, 195.6 OHV, T5
http://www.ramblerLore.com

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Stingray Quote  Post ReplyReply Direct Link To This Post Posted: Dec/12/2021 at 5:30pm
Thanks Tomj,

That is exactly how I understand it.

Also when I take the m/c off to bench bleed, I'll check a few other things like pushrod length.

See picture of my brake line swap.

Beer
Joe







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Post Options Post Options   Thanks (0) Thanks(0)   Quote Stingray Quote  Post ReplyReply Direct Link To This Post Posted: Dec/17/2021 at 8:59pm
Hi All,

Firstly, thanks so much for your help!

So I've ...
  • bench bled master cylinder
  • swapped front brake line to rear port of master cylinder and connected rear brakes to front port of master cylinder labeled "REAR"
  • gravity bled all four corners
And just road tested.  I now have good high pedal with what I'd call "normal" travel.  Both front brakes now lock up under very hard braking and well before the rears.

I'm much happier Smile

I still believe the front and rear m/c bore sizes are probably different 1" - 7/8" and that was the cause of the problem.  However it is important to note that I did not dismantle the m/c to verify this.  My hypothesis is that when the fronts were connected to the smaller rear bore on the m/c, the piston in the m/c bottomed before building full pressure (because it did not displace sufficient volume) leaving only the rear cylinders connected to the larger bore on the m/c to do the work.

Anyway, onto the next job ... replace the rotted exhaust, manifold to tailpipe, in an effort to make this car roadworthy.

Have a great Christmas holiday season!!

Beer
Joe
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tomj Quote  Post ReplyReply Direct Link To This Post Posted: Dec/17/2021 at 9:48pm
Nice! Congratulations on the research and outcome. 50 years is a lot of opportunity for oddball "fixes"!

Sorting things out has it's own rewards.

1960 Rambler Super two-door wagon, OHV auto
1961 Roadster American, 195.6 OHV, T5
http://www.ramblerLore.com

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