I need a bit of brake know how and help. I just completed the rear disc conversion on my 75 F350. The discs in the front are dual piston, and about 6 years old, but with new pads. I replaced the master with oem rebuilt, deleted the 35 year old proportioning valve, and installed an adjustable proportioning v. for the rear system. I also ran new hard line to the rear, and new soft lines at 4 corners, and center rear.

I've read 100 times all the probs people have the Caddy calipers I have. So If you just want to tell me how bad they are, pls don't bother.

I have bleed every thing well, even took off the rears and tapped with rubber mallet as others recommended. Still, I don't have the stopping ability I should have. I have never been able to lock up the brakes, and still can't. Even though the prop. valve for the rears is set at full force, I can't lock them up. Shouldn't I be able to skid? Shouldn't the fronts, at least, lock up?

I noticed the front cals were full of rusty colored brake fluid, and the bleeders were clogged. But even when fronts were new with rebuilt cals., I have not been able to stop as quickly as possible in emergency situations. Any advise is appreciated.

 

Sounds like its time to hook up a brake pressure gauge. I also think this may shed some light on your thread about master cylinders. If you went to rear discs, your stock master cylinder isn't going to hold up, because it can't deliver the fluid demands or the pressure needed to clamp down on the rotors. Without knowing the full extent of your project, I'll say that removing the proportioning valve probably wasn't a good idea, but you'd have to replace it anyway because now you're going to need more fluid going to the rear brakes than the OE prop valve could supply. You're probably going to have to do some experimenting with prop valves and master cylinders, but I'd start by comparing bolt patterns with what you have and a master cylinder from an F-450 or F-Superduty from the late 80's to mid 90's.

 

"fairlane68" nailed it, need a different MC, it takes more pressure for discs.

 

I would not have gotten rid of the factory prop valve. It shuts down the system, front or rear when it detects a leak. Now if there is a leak you will loose all of the brakes.

 

Regarding the Prop. valve: With two p.v.s I had on the truck, it never shut down the system when I had a leak. I doesn't work that way. It lights an idiot light when the fluid is depleted.

I am looking at using the mstr. and booster from a 2000 F450. I will compare at the parts store, and then will know more.

First, before I invest more $, I am going to change from the organic pads in the rear, and try a more modern pad. That may grab better.

 

Also, the E in ebrake stands for emergency.

 

What?!? No it doesn't. The proportioning valve shuts down nothing.

 

Is anyone using the m. cylinder/booster from a 2000 F450?

The master is designed for 4Xdisc, and has a 1 3/8" bore. That is 1/8" larger diameter than what I have. Also, the booster is dual 10 3/4" diaphrams, two inches larger diameter than mine. That should give me max stopping power.

Once I measure diff. in a.parts store, I'll know if it is something I can do.

 

You need to be looking at the system on the early 80's to mid 90's trucks. They had disc brakes, but they aso had hydroboost, or some of them did. A larger master cylinder bore doesn't mean more pressure. I believe it's the other way around, meaning a smaller bore is what you might need. I just set up a set of Wilwood triple master cylinders/pedal assy on my Mustang, and I had to get help with what the right size master cylinders I needed to have. I ended up using a 1" on the front, 7/8" on the rear. Find out what size your truck's MC is, and compare it to what you're looking for. You can always call Wilwood and ask them for advice. Based on what some guys here have done, a hydroboost system may be the best option.

 

In my opinion, I should feel in total control of the truck, and that includes stopping. I have never felt that. Even now, after the disc swap, it stops ok, but if a child ran out, I should be able to stomp on it and lock up or not, it should throw me forward and stop! WHY IS THAT NOT HAPPENING?

I know I need to adjust the rear calipers, but what am I missing. I have new master and dual 9 3/4" booster, new lines, adjustable prop.valve for the rear, and still it stops like ... ok, but not full on, grabbing, in control.

And by the way: The ebrake cables, etc., took a lot of adjusting, cutting and welding the cable ends, modifying, but they clamp strong now, better than the oem ever did. But it takes a lot of work to acheive.

 

Sure it does, thats why it's sometimes called the "safety valve ". If it doesn't trip, it's stuck.

 

When one side (front or rear) of the braking system develops a leak and loses pressure, the high-pressure side (that doesn't have the leak) moves the pin in the middle of the proportioning valve to the failed side. The pin has a detent that causes the switch to turn the light on when the pin moves off-center.

My understanding was that when the pin moves toward the failed side, it also blocks off pressure to the failed braking circuit so that fluid is only diverted to the side without a leak. Fortunately, I haven't been in this situation so I can't verify it. The pin does stick, so it's possible that you won't see it happen on a 30 year old valve. I could be wrong though - like I said this is just my understanding (along with mark's), but I haven't seen it happen, so I could be wrong (please correct me if so).

I think the master cylinder is still supposed to be able to apply pressure to the "good" side even if pressure drops in the "bad" side, so it would make sense that the proportioning valve doesn't need to do anything in this situation other than turn on a light. Let me know what you all think.

 

A simple Google search on prop valves will clear it up for the non believers. If it is working right you need to pump the brakes fast in a panic situation to get it to trip, thats why you should always pump the brakes slow when bleeding so you don't end up tripping the valve.

 

Thanks for the help. I may call them. They are the experts. I just finished dialing in the new Caddy calipers. The ratcheting ebrake is a bear. With the adjustments I made, and driving to find proper setting for the adjustable proportioning valve, I now have good, high firm pedal. I found where the rears lock up in a panic, and backed them off.

Still, I feel there should still be better stopping ability. Since the fronts are about 7 years old, they may not be using both pistons, so I may throw a set of rebuilts on, or rebuild them myself. My master has a 1 1/4" bore, by the way.

 

Do you know what your pressures are for the fronts? Assuming everything is working like its supposed to, the front brakes should get the most pressure and do most of the stopping. Technically they should be the first to lockup.