Noticed a hissing sound when brake pedal depressed, 79 f250 - power assisted brakes.

 

u have a vacuum leak.

 

Any ideas where to begin to look for the leak?

 

i am pretty new to explorers so i can take a stab at this....

i had a 83 marquis before i got my truck and there was a very common problem that sounded like this. If you can hear the hissing and it sounds like its inside the cabin check under the dash at the base of the steering column. Check for any vacuum lines right there and see if you can see where any leaks might be coming from.

In the marquis specifically there was a vacuum valve there to pop the e-brake when you put the car in gear if it was left on...this valve always broke and would cause the same thing. Check for something similar.

 

The vacuum booster for your brakes. It's the large round black unit behind your master cylinder. Easy to do. Remove the master cylinder , you can leave on the brake lines, remove the booster and replace with a rebuilt one. Couple of hours of work and your good to go.

 

yeah thats a lot of work to do before he even knows the booster is bad.

to check for a vacuum leak do the following...


start the truck and pop the hood (in any order you see fit)

take a can of WD40 and spray around the vacuum lines and fittings on the brake booster...if the idle surges then you have a leak there...if not it oculd still be a leak...only inside the drum....This WD40 trick works for most vacuum lines...

 

If he hears the leak when pressing the brake pedal, then it's the brake booster.

 

Best place to find brake booster?

 

under hood, driverside firewall...looks like a big black cylinder like 10" in diameter.

 

At a parts store, $80-$100 with exchange.

 

guess we covered both capacities of that question...

 

Thanks for the help! You saved me alot of guesswork.