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MG TD TF 1500 - Brake system pressure

Anybody happen to know what magnitude of pressure is generated in the brake system under normal braking? I'd like to quantify the function of a master cylinder, but I have no idea of what psi range is needed? Tnx Bud
Bud Krueger

wow, good question.

Looking at the math. .625 bore (=.3" square)with a lever arm that approachs at least 1 foot in length. If you can apply 100lbs of force that is 100 lbs feet of force on .3" squared, or extrapolating that out would be 3,333 lbs per square inch I believe.

I'd imagine in a panic situation it would be more.

And I'm only guessing at the MC bore size and length of the brake arm.
L Rutt

Hadn't really considered looking at it from the input side. The m/c bore is 7/8", about .601 sq. in. Must go and do some measurements on the pedal assembly. Bud
Bud Krueger

Okay -- within reasonable accuracy, the distance from the brake pedal to the pedal shaft is about 9 inches. The distance from the pedal shaft to the brake actuating rod is about 2 inches. This equates to a mechanical advantage of about 4.5. The area of the master cylinder piston is about .6 square inches. So, using Mr. Rutt's figure of 100 pounds of force applied to the brake pedal, we would have 100 * 4.5/.6= 750 lbs/ Interestingly enough, the diameter of the piston in a TD wheel cylinder is also 7/8". Some brake pressure gauges that I've found by Googling go up to 2,000 psi. Now, where can I find an inexpensive 2,000 psi gauge with the right adapter? Hmmm ? Bud
Bud Krueger

Bud oxygen tanks are 2000PSI or greater ( gauges are up to 2500 psi) I have anumber of gauges in various states of disrepair whose pressure gauge measuring aspects are still functional come up and take your choice or a bunch to cobble something together ( Glad to return all your favors)
Jon Levine

Pretty interesting. Is the M/C output divided by 4 (for each wheel)?
Peter Dahlquist

Volume output is divided when applying the brakes, but pressure same on each piston and lines throughout the entire system. Remember the fluid is not compressable and is totally contained
George Butz

The pressure is above 2500 #/in2. That pressure is OK but much above that and the top of the brake switch will blow off.

I worked at FASCO in Rochester NY right after the war. And that was where the power brakes in the Cadillac's were found to blow the tops off the brake light switches in a panic stop. Which started the shift to switches that were not in the hydraulic system.
Bob Jeffers

Brake pressure = L x R / A
where L = leg force, R = pedal ratio, and A = piston area.
L = usual strength x Panic factor, which could easily amount to 3-500 lbs. Giving a total pressure in the 3500psi range.

Hydraulic systems commonly operate in this range, or even in the 5000psi range, so gauges should not be too hard to find. Good ebay surplus item, or if you have a press or Portapower unit.

FR Millmore

Yikes, FRM. I'm just trying to compare different seals in a master cylinder, not after panic stop numbers. The pedal pressure that seems to be used in testing is 150 pounds. In our case that yields about 1,000 psi. Bud
Bud Krueger

Well you didn't specify your needs!
It does tell you something about Bob's story, and maybe why rusty (or copper) pipes and improvised hoses might not be a good idea. I've always used 1500 as a good "normal max". number.

FR Millmore

You're right. See 'Tight m/c rebuild' Bud
Bud Krueger

There are a bunch of 2000 psi gauges on this page in the $6 range.

If you want higher pressure gauges go to the next pages.

Jim B.
JA Benjamin

This thread was discussed between 15/11/2011 and 18/11/2011

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