Brake pressure standards V6 CJ5

So is it standard for the rear brakes to have more pressure going to them than the fronts on the V6? I took it the 5 to a break shop to get the brakes fixed up and they said that they noticed the previous statement and thought that it was strange. It has a dual M/C that had work done to it prior. Any ideas?

 

What year is the jeep?
Does it have disk or drum for the front?


With that being said it could depend, there's been allot of discussion on this forum about the duel master cylinders.
But anyways the front part of the master cylinder should go to the rear brakes and the back part of the cylinder should go to the front. In addition to this you say it had previous work done the residual pressure valves could be backwards or wrong one. These are my guesses, how much of a pressure difference is there anyways?

 

Drums all around. 69 CJ5. I dont know how much pressure, took it to a break shop. Could see if someone switched the lines.

 

since you got drums, and you got a 1969. The only other thing I can think of is if someone swapped out a master cylinder from a later model cj5 that had disk brakes, which would change residual pressure valve to 2 lbs instead of 10 lbs which is used for drums.


i'm kinda wondering I should of asked this, is this residual pressure were talking about? or actual brake pressure like when you press the brake pedal and apply the brakes?

 

Actual pedal pressure. Pressure comes out of the rear just fine, thinking it my just be a cup has gone bad in the M/C.

 

Is this one of the orginal step bore Master cylinders?
If it is then you should have different pressure in the front/rear systems.
Think about it: Pressure is measured in PSI for both front and rear.
If the back cup/bore is larger diameter than the front cup/bore, the larger cup/bore will have a lower pressure in PSI than the smaller cup/bore.
The same pressure is applied by the pedal to both cups.
The larger cup will have the pedal pressure applied to a larger area, hence lower PSI than the smaller cup.
The smaller cup will have the same pedal pressure applied to a smaller area, hence a higher PSI than the large cup.
The smaller bore/cup will be connected to the front system, applying higher PSI to the front brakes.

Make sense? Kind of an elegant solution to the problem of generating different pressure to front/rear without using a proportioning valve. Downside is it must be more expensive to manufacture. These original step bore master cylinders have become very hard to find in good condition.

Don

 

Larger piston equals more psi on the caliper or wheel cylinder piston but corresponding loss in movement. Larger piston at the wheel cylinder or caliper equals more force on the shoes or pads with a corresponding loss in movement. Smaller piston equals more movement but corresponding loss in pressure and force. Pascal's Law.


Edit: just re-read. You were talking about master cylinder bore diameters, not wheel cylinder or caliper bore diameters. You are dead on. My apologies. I'm leaving this up though for info for future searches. E-beer to you!

I'd also like to add that later CJ's used a straight bore master cylinder but modified pressures by having larger bore wheel cylinders on the front (11" used 1 1/8") and rear 11" were smaller (1" iirc).
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To add what Steve said, the real purpose of the rpv is to hold a slight pressure on the lips of the cup seals in the wheel cylinders to keep them from leaking bRake fluid and losing pressure when not applied. Systems that use cup expanders in the wheel cylinders do not use residual pressure valves typically as they are unnecessary.


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Found out that one of the cups was just split to no end.... sprung for a master from a 71 for 30 which was the same as getting a rebuild kit for the one I had. Now he's back on the road!