As I mentioned, having to pump the brake pedal multiple times is common when rear drum brakes are out of adjustment. The first pump pushes the shoes out part way to the drums, then they retract relatively slowly compared to how fast the brake pedal comes back. The master cylinder takes a new bite of fluid, then you push that additional fluid out and it makes the shoes travel the rest of the way to the drums. Once they make contact, pressure builds up quickly as you push the pedal further. The clue is the pedal will remain high and hard as long as you hold pressure on it, and once you release the pedal for at least five seconds, the shoes will fully retract which pushes all that fluid back into the reservoir, then the next time you push the pedal it is low again.
Since you have rear calipers, we have to look for the next most common cause of low pedal, and that is air in the system. You will not be the first person to switch left and right calipers, but the problem started before you replaced them, so that can't be it.
By the way, we used to rebuild calipers and wheel cylinders with every brake job. Today most schools don't teach that valuable skill, and we just replace them with professionally-rebuilt units, Cost has come down a lot too, so it makes economic sense to just buy rebuilt calipers. What do you do if the auto parts store only has a left caliper in stock, and you need a right one for a customer who is from out-of-town and on vacation, and it's late Friday afternoon? In the case of many Ford and Chrysler products, to save money, the caliper castings are all the same, and only the location of the bleeder screw is different. It is not unheard of to put a left caliper on the right side, but to do that, you simply need to crack lose the hose connection and hold the caliper with that as the high spot to bleed the air through that. Once the air is bled out, tighten the connection and install the caliper on its mount.
Another thing to consider is the effect of a lose wheel bearing. This doesn't apply to front-wheel-drive cars because for a bearing to be this bad, there would have been much more serious symptoms before it got that bad. Rear-wheel-drive cars with tapered front wheel bearings, and four-wheel-drive trucks with tapered bearings can develop a low brake pedal if there is too much play in them. While driving over bumps and turning corners, the brake rotor will wobble and push the piston back into the caliper. You need to push the brake pedal once to run the piston out until the pads contact the rotor, then you usually need to release the pedal, then take another stab at it to apply pressure to the pads.
Now that I've described that, consider that a warped brake rotor, front or rear, will do the same thing. Sometimes you can get a clue by observing how long it takes to act up again. With lose wheel bearings you typically have to drive a considerable distance, as in a half mile or more, before the piston gets pushed into the caliper housing. With a warped rotor, on a Chrysler product you only have to drive a few yards, basically one wheel revolution, but it has to be at a substantial speed. The mounting system on Chryslers allows the calipers to slide left and right fairly easily so at low speeds one will just move with the warp in the rotor instead of pushing the piston into the caliper. At faster speeds the momentum needed to get the caliper sliding is too high and it's easier for the piston to get pushed in.
On Ford trucks the calipers used to be mounted very tightly and were only expected to slide a very tiny amount under hard braking pressure to adjust for outer pad wear. That's why you would always find the inner pads worn down to metal while the outer pads were in good shape yet. A warped rotor is not going to force a caliper to slide, so the piston will get pushed in instead. One wheel revolution is all that's needed to push the piston in and cause a low brake pedal the next time it's pushed.
The clinker here is you will not always feel a warped brake rotor. "Thickness variation" will be felt as the brake pedal pulsing up and down once per wheel revolution. That will cause the piston to retract too, but it's the pulsing pedal that is the common complaint. A rotor can also have "lateral run out". The thickness is the same all the way around, but both plates are moving left and right as the rotor rotates. That almost always will not be felt in the brake pedal unless it's very bad, but it can cause a shimmy sensation in the seat and armrest.
Thickness variation is found by measuring the rotor in four to eight places around its friction surface, with a micrometer. There is surely some spec. But if there is any variation, a light cut on the brake lathe will take care of it. Lateral run out is found with a dial indicator. This happens most commonly to new rotors, and again, a light cut on the lathe will solve it. It is also real common with new Chinese rotors. There's nothing wrong with their quality, but when we make parts from cast iron, we set them aside to "age" for 90 days before they get their final machining. The Chinese cast 'em, machine 'em, pack 'em, and ship 'em, then they age on your vehicle. Minor warping is common. The proper fix is to take a light cut, typically after about three months. Cranky customers who insist on new rotors under warranty can expect to have the same problem in another three months.
Now that I've described that, it will be easier to understand where a lot of do-it-yourselfers run into trouble. This does not apply to rotors that sit on tapered bearings, like all cars had in the '60s and '70s. I'm pretty sure it also doesn't apply to your truck, but please allow me to explain the problem so you watch out for it. It applies to rotors that have a large center hole and slide onto a hub that is part of the wheel bearing. I had to use a photo from Rock Auto for a '92 Chevy Cavalier to make my wondrous point. If you look at my nifty red arrow, it is pointing to one of three access holes used to get at the bearing mounting bolts. Water can splash up there and cause three corresponding circles of rust to form on the backside of the rotor. If that rotor is removed, rotated one-fifth of a revolution, then reinstalled, those rust spots will be trapped between the rotor and hub surfaces, and that will cause lateral run out. The rotor will wobble and can push the piston into the caliper housing. There's your low brake pedal again the next time you apply the brakes. Worse yet, when a rotor is machined, those rust spots must be scraped off, AND the ring of rust corresponding to my pretty blue arrow must be scraped off. The mounting cones for the lathe sit against those areas, and if the rust isn't cleaned off, those high spots will make the rotor sit not squarely on the lathe, and a lateral warp will be machined into it. Many do-it-yourselfers who replace their own wheel bearing don't realize those rust spots are there, and they end up with a pulsing brake pedal. The rotor not sitting squarely on the hub causes the wheel to wobble too, so you'll have a shimmy in the steering wheel.
Now that I've pointed that out, be aware also that rust chunks and scale can break off while you're reinstalling a rotor and get stuck between the hub and rotor and cause a pulsing pedal or a low pedal like you have. Stuff can also get stuck between the rotor and wheel. The brakes won't be affected, but the wheel and tire will wobble. That is more common with heavily-corroded cast wheels.
As a general rule, if you can pump the brake pedal up slowly so it's high and hard, and it doesn't slowly sink away, air in the hydraulic system is not a suspect. Any air expands quickly when you release the pedal, so it can easily compress again the next time you push the pedal. When air is the culprit, the pedal will always be low except if it's trapped somewhere where fluid moves in and out very slowly, as in ABS controller chambers.
One thing I have never tried that you might consider is driving the truck and forcing an ABS stop to actuate the valves in the controller. That might release some trapped air, but it will still need time to work it's way up to the reservoir. Another trick that might work after that is to use a large flat blade screwdriver to retract both front pistons. That will push a lot of brake fluid up to the reservoir, and hopefully, any air that got released from the chambers. Remember though that you'll need to work the brake pedal multiple times to run the pistons back out, and don't push it over half way to the floor when you do that. I always retract pistons that way before I remove a caliper to install new pads. It works real easily on Chrysler and GM products. You'll have to work really hard on Ford trucks. You should never need a c-clamp to retract a piston. If you do, there's crud built up in it. Replace the caliper or rebuild it for that.
Friday, October 21st, 2016 AT 7:26 PM