i have just purchased a 1986 x-cab 4x4 4 clyinder that has some weird brakes i have perfect brakes when i back up but when i go forward my pedal goes to the floor so far i have checked to make sure the rear shoes are on correctly, replaced shoes and wheel cylinders on both sides. checked front calipers and replaced pads, replaced the load proportioning box ( the box above the rear axle on the gas tank side) replaced the master cylinder and have bled the brakes several times according to the manuel, and adjusted the brake shoes, just to double check myself i have taken parts off of a vehicle that has great brakes and put them onto this vehicle i cant find any leaks and the system appears to be air free but when i go forward the pedal goes to the floor put it in reverse and i have great brakes my tech manuel says for spongy brakes there is a check valve but i dont know were thats located i would think if i had brakes in reverse i would have them going forward if i pump the brakes i can get a good pedal going forward but in reverse they are good the first time you push them i would think master cylinder but its been replaced twice what about a booster

 

Don't know about booster but would be interesting to know what the fix is.

 

I had this same ploblem.

I was bleeding the brakes with the engine off. When I bled the brakes a second time I had the engine running and it seemed to make the difference.

Not sure if this will help but might worth a try.

 

i'll give this method a try and let you know the results

 

What sequence are you bleeding the brakes? farthest to nearest, then LSPV. You want to bleed them Drivers Rear, Pass Rear, Pass Front, Drivers Front, then LSPV. It might take a couple of times to get all of the air out of the system. When you replaced the Master Cyl. did you bleed it at the fittings? Also check your vacum lines, especially the brake booster one. Hope some of that helps out.

 

i bled the brakes with the motor running and in the recomended sequence and still no luck when i put the master cylinder on i bench bled it and then i bled it again at the fittings i checked all of the vavum lines and did find a couple of cracked vacum lines but still no luck even after repeatedly bleeding i have noticed that if i pump the brakes several times when sitting in neutral that the motor idles down but if you hold the pedal down or release it the motor idles back to normal the vehicle still has super brakes in reverse but in forward the pedal goes to the floor but pump it once or twice and you get a good pedal

 

I had the same problems and bled the brakes 3 times before I finally got good pedal feel. Try tapping (not hard) on the master cyl housing to free up any air bubbles that might be trapped in there. Dumb question but are doing the bleeding yourself or do you have someone pumping the brakes (10x's) and holding pressure while you crack the bleed screw?

 

i have used both the auto matic bleeders and the pumpimg method depending on if i have help or not i have probably bled the sytem in excess of 15 times i have great pedal when traveling in reverse but the minute i shift into first and take off when i hit the pedal it goes to the floor

 

still no luck i think today i will try a different master cylinder and brake booster

 

Do you have a hill you can let it roll in neutral both forwards and backwards? I would try to determine if it matters what gear you are in, or if it is only what direction the wheels are turning that affects the situation. Is the booster's vacuum line attatched to the motor in the correct place? This is a wierd problem indeed.

 

today i replaced the master cylinder and the brake booster the system was bled 4 times with no apparant air in the system and once again i have great pedal in reverse and no pedal when rolling forward it doesnt seem to matter about what gear i'm in i tried rolling it on the hill and i have great brakes/pedal in reverse and no pedal when going forward i also unhooked the vacum from the master cylinder and the pedal remained hard in reverse but when i traveled forward and applied the brake the pedal slowly went to the floor. when i go backwards the front brakes are working first and in forward when i finally get the brakes to catch the back tires skid in the gravel

 

Is the LSPV adjusted properly? Do you have a rubber brake line bulging from the pressure? Are you sure the rear shoes and wheel cylinders are all installed and adjusted properly? Can monkeys fly out of my butt? From what I understand, the rear brakes are designed so that the forward turning of the drum actually forces the shoes into the drum harder to help increase stopping power. If this stuff is set up wrong physics will work against you.
Was this a problem when you bought the truck, or did it develop after doing some brake work? I have traced a few problems back to my own handy work in the past.

 

brakes were like this when i purchased it lines look good and i have replaced the lspv from a working truck luckly i have 2 1986 pickups so i can go back and forth with parts from each to double check myself and the new parts i have purchased

 

i think i have my brakes fixed i removed the lspv box and replaced it with a simple three way brake line junction fitting and then bled them several times so far the pedal has maintained and i have decent brakes hope it last