Brakes: brake pressure

[doug]

If I remove a brake line from the master cylinder, cover it with my finger and have someone pump the brake pedal, will I be able to hold the pressure or will it squirt out anyway? I just figure the pressure should be higher than 10 - 20 psi.

Any guesses?

[oilnsteel]

Brake pressures can reach 1500 psi. I would consider 10-20 pounds to be inadequate.

[Mike Van Veghten]

Yea...as noted...a little higher thAn 10 or 20 psi., but you're close...
Most "normal" pressures are in the 1200 to 1300 psi range.

Mike

[StudeRich]

On the other hand Doug, if you were to open TWO Ports such as a wheel cylinder Bleeder, and Stop Light Switch Port to reduce the pressure you probably COULD hold ONE Port closed, if that would help considering we don't know the goal.

[doug]

I'm "helping" my son and working brakes. There seemed to be too little pressure from the master cylinder. Thought I would pass it by others to see if I was correct in wanting more pressure. 1 Master cylinder on the way now. Thanks all.

[mapman]

Air in the line also will substantially reduce the pressure in the system as will a bad brake hose. It is even possible for a bad brake hose to not leak. This would be indicated by soft spongy hoses.
Rob

[52 Ragtop]

"Most" brake pressures run from about 900 - 1200 PSI, much less than that, and you might as well use an old 289 block with a chain to stop!
Some of the "modern" systems will produce the 1200 and up pressures, especially with anti-lock brakes. That is why some of the newer vehicles have gone to braided stainless.
The stainless also does not expand as much as a rubber hose, thus giving a firmer pedal.

Raybestos "recommends" that rubber brake hose's should be replaced every 3-5 years! I think that "might" be a little bit of over kill, but, there are WAY too many lawyers in this country! <G>
Be sure to also look for crushed steel lines, as well as brake hose that have failed internally.

Jim

[jclary]

I wouldn't use my bare finger to in an attempt to hold pressure from a hydraulic cylinder filled with brake fluid. Depending on the ratio that involves the length of the pedal (lever), diameter of the piston, volume, it is possible to inject yourself with the fluid. I used to carry a video to use in safety meetings at my customers who used high pressure fluid systems for pumping lubricants, paints, and solvents. The results of accidents involving hydraulic fluid injections are some of the scariest, horrible pictures you'll ever see.

The risk of getting a serious injury from placing a finger over a small brake cylinder might be small...but I don't think I'd want to take a chance of finding what the tip of my finger would feel like full of DOT 3 or an infection that could follow.

How 'bout buying a small gauge and adapt it to connect to the cylinder.

[doug]

To make a short story long - we are putting a Jeep Wagoneer that was parked in the desert for 25 years (His 2nd similar project) back on the road and replaced all rubber hoses, wheel cylinders and shoes. Actually everything rubber on that thing. While bleeding the brakes with a clear hose on the bleeder I use my finger at the end to keep air from creeping back into the system. In tracing the low volume, I removed the line from master cylinder to proportioning valve and could block the pressure with my finger. I worked on aircraft hydraulic systems (3000 psi)for 40+ years (Skydrol is really nasty stuff) and have respect for anything hydraulic, and your tip is appreciated. Actually spent several hours on the proportioning valve before suspecting the master cylinder which should have been replaced anyway, everything else was.

Doug