Hello fte members,

I have a problem with the brakes on my 79 f250 ranger.

It's a 3/4 ton 4x4 with a 351m. It has the stock (vacuum) brake booster and master cylinder.

I do not know if there are any part numbers on it. or any left, that haven't rusted off yet.


When I hit the brakes I hear a big air woosh, and then the pedal hits the floor or stays an inch above the floor.


My question is how do I determine is if its the master cylinder that is bad or if its the brake booster that's bad?

And lastly kragen's mentioned if it's a single or dual piston mc? If there is a way to tell??

I believe it would be a single?????


Thank you guys for your help I appreciate it

 

Disconnect & plug the vacuum line going to the booster and see how the pedal behaves, I'll bet it's more "normal" (albeit non-power-assisted).

The whoosh you hear is pretty indicative of a ruptured vacuum diaphragm.

The following PDF document might help explain some things:
http://www.autoshop101.com/forms/brake02.pdf

Parts stores may sell a booster + master cylinder combo which I'd recommend based exclusively on the age of your truck.

You'll need to turn in the old parts as cores.

 

Thank you very much i appreciate it =)

 

Agreed, the booster is probably shot.

The single or dual reference is probably for the calipers up front, single or dual piston caliper, and not the M/C.


If you do buy a booster buy one for an F-350. They are a dual diaphram booster and are smaller in diameter.

 

thank you guys.

I undid the vacuum line to the booster, and the brake pedal came further up but was extremely stiff and would not hold the truck while in gear.

I also checked the mc and watched it while my friend pumped the brakes and saw it shooting fluid an inch in the air with the cap off

i also saw small air bubbles on the top of the brake fluid in the front cylinder


Thank you for the idea on the front calipers..the dual piston reference to the mc threw me off as well


Do you guys know how to check if the proportioning valve is off??

 

and as far as checking if the front calipers are working..to pull them off and put a c clamp on them and see if they squash down?

 

https://www.ford-trucks.com/forums/1...ing-valve.html

The C clamp on the calipers is a good test.