'86 F-350 4x4, brake pedal is very hard. Somewhat better than when there is no vacuum, but power assist is marginal. Vacuum pump is about 1-2 years old and pulling 20"+ vacuum with reasonable recovery time. System holds vacuum for quite a while sitting with engine off. Engine has been off for 20+ mins and still above 15". Replaced booster today and seems slightly better but still not what I would expect or remember from a few months ago when I last drove this truck. Booster is same brand and model as I put on my 460 truck last week (old booster on that was leaking bad, causing misfire at light load and had negligible assist), but while the 460 truck has very easy pedal effort on the brakes this IDI still take a lot of pedal effort, as somewhere between manual brakes and worse than manual brakes (poor pin location on power brake pedals). The IDI should be pulling more vacuum with a pump than a 460 with a bit of overlap on the cam.

For context I just got done swapping in a rebuilt 7.3, used the same vacuum pump that was on the old 6.9. Vacuum line goes from pump to a tree on the firewall/cowl area, out of that one large hose goes to the booster and tees off to the cruise control. When pulling the check valve from the booster there is vacuum there, feels fine by the finger test. No lines between the pump and booster are kinked. When I pump the pedal I see vacuum drop then climb. It feels like there's more assist when pushing the pedal when stopped, but when moving assist is minimal. The truck stops fine but requires monumental pedal effort to stop in a hurry from higher speeds.

Any ideas?

 

I think your vac assist booster is supposed to fall and stop when you start it. Back in the day, MD inspections required vehicle off, pump the brake several times and WITHOUT removing your foot pressure, start the vehicle. Upon starting, the pedal should fall and stop. Aside from obvious fluid and vacuum leaks, pedal going to the floor, or bypassing, I do not remember a stated minimal/maximum drop. I remember most vehicles dropping 1 to 2 inches.

I have also seen frozen wheel cylinders due to moisture and rust. Brake systems MUST be flushed since the fluid is hygroscopic. If your fluid is dark or black you are way past due.

Are your rear shoes contaminated with grease, oil, or fluid?

 

x2

That's how I test a vacuum booster and it always seems to give good results.

You could always pull the brake booster off of your gas truck and test it. Perhaps you got a bad parts when you replaced the one on your IDI.

 

The power boost from my replacement booster a year ago also didn't seem near as good as the old one.

Before I could lock up all four if I wanted to, now it still brakes hard, but has too much resistance to get that much pressure on them.

I changed the boost/master cylinder both with the correct heavy duty type, and ended up changing the vacuum pump a few months later.

I guess this is why people go hydro boost, but if I had a choice I'd try to get a 10.5 rear end with disc brakes and call it a day.

 

Have you observed how much fluid comes out of the wheel cylinder and caliper bleeders? Front brakes perform upwards of 75% or more of the breaking due weight shift and nose dive. With rear drums, the proportioning valve throttles the rears depending on nose dive and rear lift.

The proportioning valve is installed on vehicle's with front disc, rear drumbrake systems. They provide balanced braking during sudden, hard braking by restricting fluid pressure to the rear brakes. This helps prevent rear wheel lock up as the vehicle's weight is shifted toward the front wheels."