.. having the hardest time bleeding the brakes.

[homelessduck]

They WILL NOT build pressure. I swapped axles, calipers,etc. so the whole system had a lot of air in it. But after nearly a gallon of brake fluid they still won't build pressure. I have a weak stream coming from all ends, but it won't build past that. After a few pumps the pedal gets stiffer, but still bleeds to the floor. There doesn't seem to be any air in the system, and it is not leaking anywhere.

I am seriously about ready to have a scrapper come haul it away.

 

[gonecrazyi]

Did you try bleeding the rabs valve? Is it possible that the master cylinder gave it up?

 

[homelessduck]

Well, here is a little history on the truck:

It is a '75 F100 that I put a 7.3 in. I have been working on the swap for months now, and am just now getting to the brakes. I bought the truck years ago, parted it out , and started the diesel swap last spring. It has been sitting ever since. I converted the front axle to 8lug - Chevy, and put the diesel rear axle under it. I am using the same old MC that has been in it ever since I got it.

What are the symptoms of a bad MC? I've never really had to deal with one. What is a rabs valve?

Thanks!

 

[gonecrazyi]

I dont think a 75 had a rabs valve, on the late 80's and up trucks it was the device right under the drivers feet, on the frame rail, that pulses the fluid going to the rear. Basically an anti lock brake system.

I wouldnt trust that old master, there pretty cheap to replace. This could cause low line pressure as well as air entering the system.

 

[J D Recovery]

Thats pretty much what a bad master does. Try bleeding it at the master. but it sounds like the seal is leaking inside the master cyl

 

[homelessduck]

The line pressure is definitely WEAK. It isn't enough to compress the caliper at all. No rabs valve on it. The MC was working fine when I parked it.. but it was parked for awhile, and had the hood off for a few rain storms...

 

[homelessduck]

I guess I will pick up another MC..

Thanks guys.

 

[homelessduck]

I installed a new MC, and tried to bleed them again. The pedal is stiffer now, but still bleeds to the floor. Is it possible that the calipers are toast? They don't move at all... though, there is not a TON of pressure when I crack open the bleeders.

 

[comotionman]

did you bench bleed the master before install? are you getting any fluid pumped out of your bleeder valves?

 

[homelessduck]

Did not bench bleed the MC.

I am getting fluid out of the bleeders.It shoots out and drains for a few seconds.

How important is bench bleeding the MC? How do you do that..? I would assume that I need some bleeder valves or some kind of one-way valves to screw into the MC..?

 

[comotionman]

its pretty important. otherwise what is going on is you have the piston in the master moving within an air pocket and you are not getting much if any hydraulic pressure. they used to include bench bleeding kits with the master cyls, not so much anymore. all the bench bleed kit consists of is a couple of threaded plastic fittings with hose barb type nipples on them and a couple pieces of rubber hose just long enough to reach into the master cyl reservoir. its super easy to bench bleed, just thread the adapters into the brake line ports, connect rubber hoses, fill the master with brake fluid, loop hoses into the reservoir and make sure they stay submerged in fluid. then manually pump the piston until you do not see any air bubbles, then install. you can do this while on vehicle, just remove brake lines and go for it.

 

[homelessduck]

I guess I will run to town and grab some barb fittings, and bleed the MC. Blown calipers wouldn't give me these symptoms? What exactly happens when the calipers are shot? I've never had to deal with bad calipers.

The barb fittings are just pipe thread, right? Something I can snag from Ace Hardware?

 

[flatlander]

Auto zone has a brake bleeding tool. Comes with a bit of hose and what not. I used it once but it's been so long I'd have to have it in my hands to remember exactly how it works.

 

[homelessduck]

Alright, I will try the parts stores first. I really hope that is the problem. Otherwise I have NO idea what it could be, other than calipers.

 

[Danoflapper]

I'm pretty sure if your calipers or wheel cylinders were faulty you'd have brake fluid leaking everywhere. If the wheel cylinder/caliper pistons were seized you still wouldn't have any weak pressure problems. I bet you get the MC taken care of and you're all set.

Good luck! Don't you just love the gross feeling of having Brake fluid on you, that and the taste of it Mmm fun stuff!