The truck is my 1970 F100 4x2 with 300 I6, T18 trans, manual brakes, and HD 9 inch rear.

Here is a list of what was replaced: Master cylinder, all hoses, all wheel cylinders, shoes, 3 drums, and hardware. The real axle seals were also done at this time, but I don't see any relevance there. There are no fluid leaks from the brakes or the rear axles.

Ever since I went through the brakes a few weeks back it has felt odd. At first the pedal seemed a bit soft. It seemed to have to far to travel before it would engage the brakes. They stopped the truck just fine, it was just different. That sometimes happens, plus I needed to give the shoes time to get broke in. After a week of driving the truck, I went back through and adjusted all the drums again. This brought the pedal back up to where it felt like it should be. I was happy.
I was only taking short trips in the truck at the time, but I noticed it was acting odd. Once I took a longer trip I found that the more I used the brakes the more pressure the pedal had. It would build up enough pressure to cause the brakes to drag and all slack in the pedal is gone. It was enough pressure to push out the brake light switch from its position. It feels like it is the rear brakes dragging. After it sits overnight the pedal is back to normal, and I can even pull it up about a 1/4-1/2 inch so it is not that the rod is too long.

My guess is that the reman master is crap. What are your thoughts on this? Am I missing something else that would cause this?

Thanks for any and all advice.

 

That was my first thought, too. Usually they (M/C) leak internally though, and sink to the floor.

 

If you're saying that the brake pressure blew the nylon brake warning switch out of the pressure differential valve, then the seal(s) on the valve spool are bad and need to be rebuilt/replaced. Muscle Car Research has the seal kit for that valve. Under normal circumstances, there shouldn't be any brake fluid or pressure being applied to that switch.

Also, not a good idea to use a remanufactured brake MC. They have a high failure rate. Always buy a new MC.

 

The brake light switch on the pedal. it's cheap plastic that is screw in or out to adjust.

 

3X on the M/C Greg. Since you replaced the brake shoe springs we know (in theory) they are not weak. So they should be doing their job to pull the shoes and therefore the wheel cylinders back if the M/C would allow it. Sounds to me like whomever reamed the M/C when rebuilding it did something so the fluid can't go back in the M/C correctly when you release the pedal. Plus you mentioned the pedal hasn't felt right from day one. I'd take it back and warranty it. 2X Steve on getting a New one this time.

 

going to pull it Saturday and see if they will let me pay the difference for a new one over a replacement reman.

If it was just weak springs it would not effect the height of the pedal. Also just the drum rotation would force them back enough to not drag until the next time the pedal was applied.

 

The only thing I can think of in the MC that would restrict fluid return would be the RPV(s) --Residual Pressure Valve.

In the original OEM MC, there would have been an RPV in the primary and secondary ports for the drum brake circuits. However, this is a replacement MC and even though it may be a direct replacement for the particular appication, chances are very high that the MC does not have any RPVs installed behind the port seats.

RPVs can get clogged up with rust scale and debris. If this happens, it can restrict flow either exiting or trying to re-enter the MC. But as I said, there's a very good chance your MC doesn't have them.

The object that made RPVs pretty much obsolete are cup expanders inside the wheel cylinders. These can either be cup shaped metal discs that install between the seals and the ends of the springs or, they can just be a coil spring that has tight, conical wound ends to push the lip of the seals tight against the bore of the wheel cylinder.

Cup expanders don't get clogged up with debris like RPVs, they are cheaper for manufacturers to produce and they are easier to service.

Example of a cup expander for the front wheel cylinder on a '70 F100. Notice the tightly wound conical ends on the spring.

BrakeBest Hardware & Hydraulics WCK13620 - Drum Brake Wheel Cylinder Repair Kit | O'Reilly Auto Parts

 

Ultra, so you are thinking it could be the wheel cylinders?

I am trying to see how springs in the wheel cylinders would cause the issue. Could you possibly explain a little more about how that could be the case. I am just not able to wrap my head around it.

 

I was thinking more like whomever reamed the rebuilt M/C found it had to go so far that it would be to big of I.D. And then Sleeved the M/C bore. Holes in the sleeve might not be correctly over the ports in the M/C. Pressure could force the fluid past this restriction but suction can't keep up.

 

Unless there's something going on with the pistons in the wheel cylinders to keep them from retracting correctly, I don't think the problem is with them.

What I was saying about the MC is that in its factory original build condition, the MC would have had an RPV in each of the MC ports for the two drum brake circuits. The RPV (or, residual pressure check valve) is a spring loaded duck-billed device.

It lets fluid flow freely out of the port, when you step on the brake pedal, and fluid freely returns until the pressure gets down to around 10 psi. At which point the tension of the spring closes the residual pressure check valve off. This keeps a small amount of pressure on the wheel cylinders to keep the lip of the cup seals tight against the interior walls of the wheel cylinder bore. This is to keep air from being drawn in from around the seals and into the wheel cylinders.

Exploded view of a typical drum/drum manual brake MC. Notice the RPV's location.




Today, you can go to the parts store and get a replacement MC specifically for your particular truck/application but, chances are great that the replacement MC will not have RPVs in it, even if the OEM original one did. --goes back to the use of cup expanders in the wheel cylinders which made the RPVs pretty much obsolete.

IF your MC does have RPVs, there could be some rust scale or trash lodged in one or both residual valves that could be causing a restriction of the fluid returning back into the reservoirs, which could keep elevated pressure levels on the brakes, even when you aren't applying the brake pedal. Since your MC is a replacement MC though, there's a good probability there aren't any RPVs in it but, there's a slight chance they could be there.

It's very simple to check a MC for the presence of an RPV but, the MC has to be unplumbed to do the check. Take a 3/32" drill bit and gently insert the shank end of it into the hole in the port seat (where the brake line would attach). If the bit only goes a very short distance into the port, then an RPV is present. If the drill bit goes in a fair distance, no RPV is present and that would rule the RPV out as a potential problem.

This is a photo of the old drum/drum manual MC that used to be on my truck. I don't know how old it is or how long it was on there before I pitched it for a modern, less problematic, MC. You can see that I have a 3/32" bit with the shank end inserted into the primary port. The bit only went in about 1/4th of an inch and stopped. I could feel that there's an RPV in that port (I also checked the secondary port and there's one in there too). I don't know if this MC is original but, if it's a replacement, it's probably very old and was built before they pretty much stopped putting RPVs in them.

 

I went ahead and a swapped out the reman with another reman. The New will take a week to order in, so I gave this a try since they had it in stock.
So far, the replacement is working properly. the system is no longer holding pressure when it shouldn't. Thanks for all the info guys.

 

Glad you got your brakes working again. Did you happen to check the old MC for RPVs before turning it in or, check the replacement MC for them prior to putting it on?

 

Neither seemed to have the RPVs. I used a small pick and it went in a good bit more than 1/4 inch.

 

Was the pedal, with the previous MC, slow to return to the static position after you would get off the brake pedal?

Rebuilt and even brand new MCs often have small bits of debris inside them from the machining process. This grit/debris can cause the pistons to drag inside the bore.

When I get a new MC, I take it apart and wash the bore out with acetone, wipe the pistons/seals down, coat the bore and pistons with fresh brake fluid and then reassemble it before bench bleeding and then putting the MC on.

 

I am not sure what you mean by static position. The pedal returned up without issue. Actually it was usually higher than normal due to the pressure not releasing. It would take overnight for it to return to where it should be without any residual pressure. After about 3-4 uses of the brakes it would not release pressure. It would build it a little more each time you used the brakes. Eventually it was like someone was riding the brakes causing a constant drag.