Ok, I lost you there for a minute. Settle down, this is fixable.
99% of the time when someone writes in with a problem, its one of a few different things, ans we get it settled pretty quick.
But sometimes one like this comes along. Since we cannot come to you and check things out, you are going to have to do a lot of the figuring out as to what is wrong.
The only way you can do this is with knowledge. So, I am going to give you a short primer on brakes and hydraulics. I'm sure that you know some of this, but I have no idea what, so just bear with me if I cover ground you are familiar with.
Since this is a short primer, I am going to have to take some short cuts. Which means I will have to fudge the facts slightly.
First of all, brake fluid is non-compressible. Think of it as a piece of steel. When you press on the master cylinder, it pushes brake fluid through the lines and valves to the piston on the caliper. There is no "give" in this setup. If the fluid moves, the piston moves. Period.
The amount of pressure in the lines depends on how hard it is to move the piston. The harder it is, the more pressure it takes to move the fluid. NOTE: I said move the fluid, not the piston. If the piston does not move, the fluid does not move, and vice-versa.
When you press on the master cylinder, there are two pistons. One for the front and one for the back. They are set up so they can move independently of each other. When the front shoe hits the disk, the back one can continue to move if the back has not hit yet.
When you stand on the brake pedal until it stops moving, not bottomed out, just stops, then all the calipers should be pressing the shoes hard against the disks.
In your case, that is not happening. Either the fluid is being blocked by something wrong in the master cylinder, the prop valve, or a bad line somewhere, or there is some way the pressure is not developing like it should.
Air or a bad master cylinder.
Remember, fluid is not compressible.
Air is.
If there is air in the rear system, then when you move the fluid to cause the pistons to move, it simply compresses the air bubble down smaller and you do not get enough pressure to the pistons to cause them to move.
I know you have been very diligent about bleeding the brakes, but if the air bubble is in a high point in system that will let the fluid flow by it, you could literally pump gallons of fluid through and never get it out. That is why I was so picky about the location of the bleeder screws.
When they are on the bottom of the calipers, the air bubble stays on top and you can never get it out.
Try this. Turn off the truck, and hit the pedal a few times to bleed off all the vacuum out of the booster. Then, get to where you can see the fluid in the master cylinder. Have someone press the pedal down hard and fairly fast and hold it. Then suddenly let it go. If you see a geyser of fluid erupt out of the master cylinder, then you have air on that side. Somewhere.
Some setups will let small little blips of fluid pop up when you do that, but a good air bubble is a geyser.
If you see a geyser when he presses it down, then the master cylinder is probably bad.
Remember, the master cylinder is a dual piston set up. The piston that pumps fluid to the front could be fine and the piston that pumps to the rear could be totally shot.
In fact, a sad lesson I have learned over the years, is that usually when you bleed the brakes on a system that has been used for a while, about 70% of the time, you will have to replace the master cylinder shortly.
The reason is, that for maybe years and thousands of miles, the MC pistons have been traveling the same distance in the bores keeping that part polished.
The part that the piston never goes to gets crudded up or corroded.
When you bleed the brakes, you let the fluid out until the pedal bottoms out. This causes the rubber seal on the piston to scrape through the unused part of the bore and damages it.
The thing you have to determine, is this. Why are my rear calipers not getting enough pressure to move?
Air bubble, blockage, or not enough pressure being developed in the first place.
Hang in there, its fixable.
J.
99% of the time when someone writes in with a problem, its one of a few different things, ans we get it settled pretty quick.
But sometimes one like this comes along. Since we cannot come to you and check things out, you are going to have to do a lot of the figuring out as to what is wrong.
The only way you can do this is with knowledge. So, I am going to give you a short primer on brakes and hydraulics. I'm sure that you know some of this, but I have no idea what, so just bear with me if I cover ground you are familiar with.
Since this is a short primer, I am going to have to take some short cuts. Which means I will have to fudge the facts slightly.
First of all, brake fluid is non-compressible. Think of it as a piece of steel. When you press on the master cylinder, it pushes brake fluid through the lines and valves to the piston on the caliper. There is no "give" in this setup. If the fluid moves, the piston moves. Period.
The amount of pressure in the lines depends on how hard it is to move the piston. The harder it is, the more pressure it takes to move the fluid. NOTE: I said move the fluid, not the piston. If the piston does not move, the fluid does not move, and vice-versa.
When you press on the master cylinder, there are two pistons. One for the front and one for the back. They are set up so they can move independently of each other. When the front shoe hits the disk, the back one can continue to move if the back has not hit yet.
When you stand on the brake pedal until it stops moving, not bottomed out, just stops, then all the calipers should be pressing the shoes hard against the disks.
In your case, that is not happening. Either the fluid is being blocked by something wrong in the master cylinder, the prop valve, or a bad line somewhere, or there is some way the pressure is not developing like it should.
Air or a bad master cylinder.
Remember, fluid is not compressible.
Air is.
If there is air in the rear system, then when you move the fluid to cause the pistons to move, it simply compresses the air bubble down smaller and you do not get enough pressure to the pistons to cause them to move.
I know you have been very diligent about bleeding the brakes, but if the air bubble is in a high point in system that will let the fluid flow by it, you could literally pump gallons of fluid through and never get it out. That is why I was so picky about the location of the bleeder screws.
When they are on the bottom of the calipers, the air bubble stays on top and you can never get it out.
Try this. Turn off the truck, and hit the pedal a few times to bleed off all the vacuum out of the booster. Then, get to where you can see the fluid in the master cylinder. Have someone press the pedal down hard and fairly fast and hold it. Then suddenly let it go. If you see a geyser of fluid erupt out of the master cylinder, then you have air on that side. Somewhere.
Some setups will let small little blips of fluid pop up when you do that, but a good air bubble is a geyser.
If you see a geyser when he presses it down, then the master cylinder is probably bad.
Remember, the master cylinder is a dual piston set up. The piston that pumps fluid to the front could be fine and the piston that pumps to the rear could be totally shot.
In fact, a sad lesson I have learned over the years, is that usually when you bleed the brakes on a system that has been used for a while, about 70% of the time, you will have to replace the master cylinder shortly.
The reason is, that for maybe years and thousands of miles, the MC pistons have been traveling the same distance in the bores keeping that part polished.
The part that the piston never goes to gets crudded up or corroded.
When you bleed the brakes, you let the fluid out until the pedal bottoms out. This causes the rubber seal on the piston to scrape through the unused part of the bore and damages it.
The thing you have to determine, is this. Why are my rear calipers not getting enough pressure to move?
Air bubble, blockage, or not enough pressure being developed in the first place.
Hang in there, its fixable.
J.
