I have a 84 f-250 4x4 6.9... I had a stiff (hard to push pedal,like the vacuum booster is not working,but it does) braking problem..(brake light came on)... So I decided to replace (almost everything)... New rear wheel cylinders,front calipers,vacuum booster,master cylinder, and also pads... I didn't replace lines or the differential valve ...I bled it till the cows came home, and have no air in lines( brakes are hard not spongie ) STILL THE SAME PROBLEM.... The brake light comes on, but if you press on the pedal to center the differntial valve the light goes out and stays out, but once you push the brake (hard to push) again, the light comes bake on...
The brakes work fine, its like there's no power brakes.. And the last vacuum booster did this, and now the new one is to.. So i don't think its a vaucuum booster problem..... HELP

 

Hi and welcome

It sounds to me like your vacuum pump isn't working.
The 6.9 has a belt driven vacuum pump that supplies vacuum to the brake booster. If it's dead it will be just like pressing the brakes with the engine turned of. The pedal will be very hard to push.

 

Sorry I forgot to mention I put a new vacuum pump in too... checked the vacuum and its working good... I'm wondering if it could be the differential valve... It has a plug on the top with a purple and a purple/black wire... this plug corroded off and the wires are disconnected, but the brake warning light still seems to funtion... where is the brake warning light hooked into the brake system???? And what is the plug for on top of the differential valve???? This is a "over 8500 lbs" brake system, my manual only shows a standard f-250 differential valve and it is completly different ... It just bugs me.....

 

Hi,

It's been a long time, but did you ever figure out the brake problem on your truck? Mine is doing the same thing, and I've replaced many parts. It has improved some, but I still get the incredibly stiff brake pedal sometimes. I have an aftermarket brake system, too, that was installed by the guy who owned the truck before me to tow his RV trailer. Please let me know if you found anything out.

Ken

 

It turned out to be the vacuum booster... Must have been a flawed one, cuz put a new one in and all problems gone... Brakes have never been better... I should have known, but the one in it wasn't that old..