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Brake Line Hydraulic Pressure


Asciidv

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brakep.JPG

 

I have been having problems with the brakes on one of my Dennis's and I decided to check the brake line hydraulic pressure. With the engine off (no servo action ) and an 84 year old ( no not me!:-D ) pressing the pedal down as hard as he could we saw around 400 PSI in the hydraulic line directly after the master cylinder. Do you think this seems reasonable or on the low side?

 

I know that there are half a dozen variables which could affect the result. Strangely there does not seem much data out there on which to make a comparison.

 

Barry.

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I have been having problems with the brakes on one of my Dennis's and I decided to check the brake line hydraulic pressure. With the engine off (no servo action ) and an 84 year old ( no not me!:-D ) pressing the pedal down as hard as he could we saw around 400 PSI in the hydraulic line directly after the master cylinder. Do you think this seems reasonable or on the low side?

 

I know that there are half a dozen variables which could affect the result. Strangely there does not seem much data out there on which to make a comparison.

 

Barry.

 

Hi Barry,

Output pressure on brake master cylinders are very rarely specified. If this was taken on master cylinder output, and no leakage past the piston to the open end, then it has to be good. Most importantly is that it is a servo system and working pressure will be when servo is actuated. I assume this is a Clayton Dewandre servo unit? Maybe you have problems with the servo valve or piston seal?

 

regards, Richard

Edited by Richard Farrant
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As Richard says, with no assist, m/c output pressure is simple mechanics. With a typical pedal ratio of 5:1, an pedal force of 80 lb (not bad for an octogenarian), and a m/c bore diameter of 1", you get 315 psi. So what you are seeing is reasonable. But it takes somewhere north of 1000 psi to get decent braking in my experience. That's where the servo assist has to step up.

Malcolm

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Next time when I am down at the shed I will get my tame octogenarian to press on the brake pedal with the engine running to see how much extra boost the Clayton Dewandre gives. I have seen some car brake discussions where they talk of 1200-1500psi being the correct pressure.

 

When working on brakes single handed the best thing that I have ever used is a device called "Easy Bleed". This just pressurises the master cylinder reservoir at around 20psi. and gives power bleeding.

 

Barry.

 

easybleed.JPG

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An ideal tool the Gunson as you don't need anyone else to keep topping the master cylinder up. There is however if you have a compressor a vacum bleeder as well. This has advantages on large systems. The main problem with the Gunson is you need a cap to fit the master and it can be a pain to seal. The vacum bleeder fits over brake cylinder nipple. 416RugZ%2BgRL.jpg

Edited by Tony B
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Research into pedal effort and line pressures shows a huge variation; for example, pedal efforts as low as 30 lb and as high as 500 lb. However my understanding is that typical line pressures are in the range of 800 to 2000 psi depending on system and pedal pressure, and typical design pedal pressures are in the 20 - 120 lb range for an assisted system (maybe at the higher end for trucks).

 

As Richard says, on an assisted system you are likely to be reliant on the servo to attain full line pressure, and if you are measuring output pressure at the master cylinder, fluid is not bypassing the master cylinder pressure seals and you have no trapped air in your test set up it's difficult to see what could be wrong hydraulically; so if your pressure is low you are looking at a mechanical fault with linkage, servo or master cylinder.

 

Hydraulic pressure must be equal throughout the system, so if you are measuring line pressure by tee-ing into the system at a joint (and therefore testing the whole system pressure) low pressure might be due to a leak or failing seal anywhere in the system, so your test may not be very helpful unless you can selectively exclude different parts of the system.

 

You haven't mentioned what the actual problems with the brakes are - is this more of a point of interest?

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