Hi!
I had to move my Truck today. Has been sitting for 6-7 month due to Sheet Metal Work. Brakes worked good last year. Today Brakes felt very strange and I took it to a short test drive (without doors and hood!!!) On Pavement in low speed one Wheel locked way to fast. With a little more speed Brakes felt some better and not that sensitive but still far from good. I drove to a snow-covered area and tested in very low speed. This time it felt almost normal because the low friction made all 4 Wheels locked. Jumped out and checked tracks from all Wheels. I was thinking one Caliper was seized but could it really cause this sumptoms. I was also thinking vacuum to the Brake Booster? I lost my vented Cap for the Valve Covers awhile ago and bought a new. Could this affect the vacuum. Are this Trucks equiped with a Proportioning Valve? Suggestions how to troubleshoot are very much appreciated.

 

Brakes are probably rusty from sitting for so long, chances are they will clean themselves when you start driving it more. If not pull apart and clean and relube slides. Clean rear brakes also.

 

I don't know if you have an F100, F150, F250 or an F350 (?).

If you have an F100/F150 (or an F250 up to 6,900# GVW), you should have a cast iron Kelsey-Hayes brake valve. The valve assembly will contain a metering (hold-off) valve to the front disc brake circuit, a pressure differential valve between the front and rear brake circuits and a proportioning valve going to the rear drum brake circuit.

The inner workings of the cast iron Kelsey-Hayes valve assembly looks like this.




If you have an F250/F350 that's over 6,900# GVW or greater, you should have a brass Weatherhead brake valve. It's configuration and inner workings looks like this.

 

Prolly a lil grabby from non-use...

 

I hope so too. For now I am just going to change fluid and bleed the Brakes. This way I can at least get some movement in the Brake Parts without a new test drive. (Cold without doors!!) Thanks for assistance!!!

 

I forgot to ask. Are problems with P-Valves common?

 

Ride the brakes a bit to shine up the drums and rotor........Probably all rusted up from sitting

 

They might be, seeing as how the ones on these trucks are pretty sophisticated (by 1970s standards) little devices, and have had ample opportunities to sit unused for long periods of time, leading to corrosion and crap like that.
There appear to be seals and O-rings in them, so each of those is an opportunity for leakage.
Also looks like there are some springs in there, which each present an opportunity for loss of tension, or I suppose, breakage.
Then there is that main shaft inside, the one with the notch which moves off-center due to pressure differential, when there is a fluid leak, causing the warning light switch to power up your brake warning lamp on the dash. I would assume that these shafts can seize into place, which means that you will never get a warning in the event of a leak. I think mine may have that problem, as I had a leak for a few days, and no light. I know my switch is good and in place, and that the bulb in the dash is good. And I also found the rear-facing pistons on both wheel cylinders to be seized in their bores, when I replaced them last week. The front-facing ones were free.

 

I have a new brake question today! I was bleeding the Brakes today. I wanted to change the Fluid and exercise the Brakes at the same time. Started in the rear (both sides) and everything looked normal. When continued at front (Pass-Side) and suddenly there were no Fluid coming out from the Bleeding Valve. (Reservoar was never empty). Then closed the Bleeding Valve and there were no pressure to activate the brakes. I Went to Driver Side for bleeding and everything was normal. Then I went back to Pass-side and now I got some Fluid out and pressure to activate the Brakes. Was this because the P-valve closed the right front outlet? I was thinking maybe the P-Valve "protected" the Brake System if there's a leakage?
I suppose this was not symptoms for seized Calipers or am I wrong?

 

You need to open the metering valve on the prop valve, in order to bleed the fronts.
Under 6000 lbs. GVW needs the valve pulled open, and the over 6000 lbs. GVWs have a valve which pushes open.

If the valve is closed nothing will come out. It's possible that the metering valve could be half open, exposing only one side, causing only one side to bleed forth. My truck has a pull valve, and it needs to be pushed in after bleeding because it no longer returns on it's own.

 

Thanks a lot meangreen92!! Should have asked before or read a Manual !!!
Very good Pictures of the Valves. My Truck has 6150 GVWR on the Door Tag. (Do you mean push the Valve for Trucks over 6000 or over 6900?)
I will push (or pull) the Metering Valve and repeat the bleeding process. Then close the Metering Valve after bleeding rear and front. Thanks!!!!