i bought the truck (89 f250 7.3idi) a couple days ago and they guy i bought it from said it had been sitting for a while which looked apparent. my question is why are the brakes fine when going fast but when coming to a stop from about 5mph to 0 mph you have to push on the brake pedal SOOOOOOOOOOO hard. also in reverse you have to do the same thing. it feels like when your driving and you shut your vehicle off and have no power breaks. any help would be appreciated!

 

sounds like

A) vaccum leak
B) dieing vaccum pump

the good pedal at high speeds makes sense b/c the engine is turning the vac pump fast enough to build sufficient vac, but when going slow it is not able to build enough vac.

You can hook up a vac gauge to be sure.

Is the red brake light on the dash on at all during these times?

 

yes the brake light is on

 

Diessel engines dont have engine vaccum to run the brake booster, so they have a belt driven vacum pump. Like 94' said, you either have a bad pump or a leak.

 

is there a way to tell if the pump is on its way out?

 

Hook a vac gauge up to it and run some tests.

1) there is a hose that comes off the back of the vac pump and that goes to a splitter. pull that hose off at the splitter. hook your vac gauge up to that hose. start engine and see what the vac pump is pulling. i don't know what exact spec is at idle but should be around 20-25 inhg. If the vac pump does not pass this test then you know for sure that you need a new pump. If the pump does pass this test then proceed to test #2.

2) Reconnect the main hose to the splitter. to eliminate the vac lines that control the HVAC controls from the equation you can discontect that line from the splitter and then hook your vac gauge up to that port. Doing this will esseintially put your vac gauge inline between the vac pump and the brake booster. start the engine. either put the gauge in a spot where you can see it from sitting in the cab or have somebody hit the brake pedal for you. hit the brakes once like you normally would. the vac should not drop WAY down. if it does then you have confirmation that you need a new pump. if the vac does not go WAY down with one pump, then pump the brakes. if you go below 10 inhg and it takes a while for it to build vac back up then your pump is close to death.

since you have an issue at different speeds it would be good to figure out some way to be able to drive the truck and also be able to have the gauge hooked up like i described in #2 and view it from inside the cab. then when your brakes do weired things you can check the vac and see what is going on.

If you want i can get you some pics of how to hook the gauge up.

 

Mine was doing the exact same thing that yours is doing. I checked the vacuum pump and it was only drawing 10 psi at idle, should be around 20. New vacuum pump cured it. The reason that your brakes are working better when you're driving faster is because your rpm's are higher which is turning the vacuum pump faster creating more vacuum.