Brake problems!

Now, this is not what you typically run into when swapping 3/4 ton brakes into the back of your XJ...
I was expecting it not to stop well after doing the chevy 3/4 ton disks on the 60 without changing anything else (master cylinder, etc.), so... I did the little o-ring trick w/ the proportioning valve. Well, the back locked up fairly easy. So... I put the o-ring back in, and they still lock up! Not quite as easily, but they do.
I was expecting to get crappy braking out of the rear w/ a stock setup. Do I need an adjustable prop. valve? Is this just because I have 3/4 ton disks in the rear, and XJ disks in the front? What's the deal?
TIA,
Billy

JeepFreak21 said:

Do I need an adjustable prop. valve? Is this just because I have 3/4 ton disks in the rear, and XJ disks in the front?

Click to expand...

Yes on both counts

Re: Re: Brake problems!

Eagle said:

Yes on both counts

Click to expand...

It makes sense, but how come everyone else has problems w/ the back not getting enough fluid?

Don't confuse pressure with volume.

Your 3/4-ton brakes are huge compared to the stock XJ rear drums, and much more efficient. Until you run out of fluid, those brakes are going to do a lot of work.

Not to hijack this thread but I've got a similar brake line question. My front line is 1/4" diameter and the rear is 3/16", I've got 1 ton front dual piston Ferd calipers and the rear are 1/2 ton Chevy. I'm using the stock m/c right now. I've removed the proportioning valve completely and will add an aftermarket one later on if/when it's necessary.

Question is, without a prop. valve does the bore of the brakeline provide any kind of flow restriction to the rear brakes? Will the smaller 3/16 rear line work to kind of restrict the flow to the rear calipers and provide more flow to the front larger 1/4 line and calipers thus providing a proportional braking effect?

Or am I just high?

vintagespeed said:

Question is, without a prop. valve does the bore of the brakeline provide any kind of flow restriction to the rear brakes? Will the smaller 3/16 rear line work to kind of restrict the flow to the rear calipers and provide more flow to the front larger 1/4 line and calipers thus providing a proportional braking effect?

Or am I just high?

Click to expand...

Not really. If everything is adjusted and operating properly, there should be very little (almost zero) fluid movement when you apply the brakes. All you're really doing is adding pressure on one end of the lines, and the fluid transfers the pressure to the other end. In any event, what you need is to proportion the pressure after the pads/shoes contact the rotors/drums and the fluid has stopped moving. The difference in tubing diameter can't help you at all once the system reaches equilibrium -- more pedal pressure just increases line pressure, it doesn't move any more fluid.

If you were dealing with an open flow situation, with some kind of fluid flowing through a line at moderate to high velicity, then the tube diameter would affect both pressure and volume. In a closed system such as the brake system, the effect is so negligible that it probably can't be qualtified.

The adjustable proportioning valves aren't that expensive. Mopar Performance has one (probably a rebranded Wilwood), Wilwood makes one, and Summit Racing also sells one under their own name.