Please help.
I'm trying to diagnose thoroughly my braking problems before I start throwing money and time at replacing good parts needlessly. I have spent hours and hours researching threads and firmly believe with time, patience and help I can figure out and importantly understand the problem.

My braking system is on a 1971 f-250 4x4 with a swapped in front and rear 1979 dana 60's. Disc front, drum rears. Brake lines are in excellent shape, with no visible leaks along lines or at fittings. The individual rubber flex hoses at each corners are also sound with no leaks or bulging. Rear cylinders and front brakes are also dry. Master cylinder reservoir leaks a little out the seal under the lid, no big deal I have just kept an eye on it. Topped up front bowl from half the other day. Front pads and rear shoes are fine upon inspection. Brakes are in excellent working order.....when they work!

So here are the symptoms: When driving, if I step on the brakes, the truck coasts to a stop and pedal goes to floor. However if I pump the brakes 3-5 times, pressure builds up and truck stops great. Than after driving again for even a few seconds that pressure is lost. When truck is parked and off, if I pump the brakes, the system builds pressure and the pedal is firm, when I start it up, the pedal pushes to the floor, turn it off and pedal is firm again. I mean, I can force it down but I refrain. Are my rear brakes adjusted you ask? Yes. That was my first thought since for the past month my pedal travel had been increasing. I adjusted them when I was done inspecting them. I put drums on first, than adjusted both left and right simultaneously(posi rear) until drums would not move, than backed them off a little till I could rotate them with pressure. I have yet to open the system up yet or bleed it. Air in the system seems to be the old go to, but I want to be sure that is the problem as these symptoms exist, before I risk potentially screwing something else up.
Even in my research I have gone down a hundred rabbit holes and found all kinds of people who have done the same. Could this be the master cylinder? Another popular culprit. Could it have an internal leak? I have eliminated the brake booster as the problem because it is dry and when I pull the vacuum hose the brake pedal gets hard. I have an aftermarket ssbc combination proportioning valve on the inside of my frame rail, Could the problem be here? Could the fact that it is extreme cold here lately affect that? Could water and road salt get in it and freeze. Last week I went through some ice into deep water, and the brake travel worsened a couple days after that...
Thanks for taking the time to read this, I believe the details will be important to the right person.

 

Sounds like the dreaded "air in the lines" problem, cause you're getting air in there somehow. All connections tight? In the picture, those lines look a bit rusty.......

How old is the master cylinder? Was it bench bled sufficiently?

Vacuum lines are good you say....how about the swivel connection to the booster and that line itself? Is it the only one on that vacuum circuit?

When you went through deep water, the shoes/pads naturally got wet. Drive a while with slight pressure on the brake pedal to heat/dry 'em out.

Could you adjust the prop valve any better for the rears?

 

I would double check the rear wheel cylinders. Mine recently did the same thing and I still didn't fix it but that's the problem. A little seepage under the rubber and air will find its way in. You did say you topped off the front reservoir. I'd be willing to be it's not from the cap seal.

 

Thanks for reply. Not sure of mc history. I haven't put a wrench on any fittings, was only thinking about visible leaks. You think I might have a constant supply of air entering the closed system just small enough to not leak fluid? Here is pics of my booster set up, not sure its what you had in mind. Also should I pull mc off of booster to look for a leak? I didnt detect any fluid in the little vacuum hose valve thingy. Also after the water incident, I had driven quite a bit, though a couple times I had to 'break out' over a couple days as the remaining water and moisture kept refreezing in drive train components. Thank you for your help.

 

Ok. I will take things apart and check under the rubbers. So, no to master cylinder? Thanks.

 

Me, personally, like to have a vacuum line dedicated just to the booster....but that's just me. I'd check those other vac lines off the T where your oil sending unit is.

Looks like the right plugged/capped off one could be cracked.....I'd recheck the vacuum lines.

PA74F250 has a good point - if water could have gotten in between the rubber seals and the wheel cylinders.....freeze/thaw,freeze/thaw..(expansion/contraction)....could have created a gaposis for air to squeeze by.

And, yes...to "no" on the master cylinder check.....for now.

 

You should positively fix the problem before driving it.

Master cylinders are relatively inexpensive. If it's leaking as you indicate as well as the pedal goes to the floor, replace it with a quality new unit.

SINCE the system needs to be bled anyway, this is also the opportunity to continue on with the rest of the brake system. Replace all rubber brake hose, rear spring hardware and cylinders, lube emergency brake cable, inspect drums, backing plate brake pad surface etc.

 

I'd suspect the master cylinder is bad and bypassing internally but another thing to check that can be overlooked is the wheel bearing play. If its excessive when you drive the rotor can push the caliper positions back in. When you stop you'd have to pump the brakes just like you just finished a brake job. A little play in the bearings is OK but if the tire moves more than 1/4" or so if you grab it at 12 and 6 and shake it when jacked up you may need to repack or replace the bearings.

 

Quote from OP: "I have yet to open the system up yet or bleed it."

The first thing I would do is bleed the brakes!!!

 

To test master cylinder, pump it up and get good pressure. Keep standing on it and if bleeding by the pedal will slowly sink to floor. If the rear adjustment is ok, my parking brake cables were not returning all the way and holding shoes away from wheel cylinder a little making you have to pump to get a better pedal. They were actually holding the parking brake lever partially applied inside the brake drum. Good luck Bruce

 

Concur..... it shouldda been the first thing done.. even before posting the Q in FTE.


Basics count.

 

Glad you adjusted the rears before posting, but bleeding the lines is almost as easy. If you don't have a helper, which I never do, google or youtube search for one person bleeding. I was able to do my whole truck in about 15 minutes and I had never done it that way before. Not even worth taking all of these other suggestions into account until you eliminate the easiest of possibilities

 

If it's not leaking externally

What would bleeding accomplish, exactly?

 

Evacuating air from the system.

Air is compressible... liquids are not.

 

Well sure. You didn't answer the question. But unless there's an external leak, say from a burst line or hose, a defective wheel cylinder - there won't be any air in the system. This is the point.

Whatever caused that condition, would not be remedied by bleeding. Again, absent any external leak a master cylinder that goes to the floor is defective and needs replaced.