Brake line psi

[kid]

The difference in pressure is the boost difference. It boosts 225% according to his numbers. If you can stand up with just one leg and you weigh 200 lbs you are generating 200 PSI. Push on the pedal hard and you are pushing 200 PSI. The pedal arm from it's pivot point to the foot pedal and from it's linkage point to the pivot point is about a 3 to 1 ratio on my F350. So I push hard and get 100 PSI at the pedal for a good stop. The 3 to 1 ratio sends 300 PSI to the MC booster. The booster multiplies that by 225% for 675 PSI. The 3/4 inch MC piston sends that 675 to a 1-1/16 piston for a increase of 200 % so in theory the brake could have 1350 PSI on it if my values for my pressure and sizes are correct. all from the original 100 pounds I applied. The gauge would read the hydraulic pressure of 675 psi.

But only if the booster works. Without the booster it would not change pressure engine off or on.

[fordman]

i'm not tryiing to be a smart guy or anything but everything you use for that axle including the master cylinder the brake block wheel cylinders shoes should be for that axle and year that the axle came off of.

another thing to check would be this. http://www.fordification.com/forum/view ... 335#149335

[averagef250]

Same here, Not sure on what the parts guy's books say, but the F-350's with the big rear brakes use an entirely different braking system. Absolutely nothing is the same. The master, proportioning valve, wheel cylinders, etc. If it just won't work, put your old backing plates and drums on the dana 70.

I have never owned a '79 F-350 4x4, but I have owned several 73-79 F-350 super camper trucks with the 70B HD single wheel rear and all the brake parts are different on those trucks from the Lighter GVW trucks.

I kept the 70 rear from one I had and sold it with a 60 rear under it with the small brakes. The 60 rear was from the same year Ford, but an F-250 4x4. The rear brakes didn't work for beans. I put the proportioning valve and master from the '77 F-250 4x4 on the F-350 and the brakes worked great.

I have never swapped a big brake rear under a small brake truck and had the rear brakes lack stopping power. They always come on too soon. That's why I suspected incorrect wheel cylinders with the later SAE brake volume standard.

[furray]

I called MP Brakes & Wagner today & they both said I have enough psi for the system.

[kid]

Without the F350MC you don't have the proper ratio for the pistons in the rear PSI on the line is per square inch from the MC but the force on the shoes is less.

[fordman]

i don't have enough info to help any more. from what i am gathering you have changed the axle and all the assorted brake hardware on the axle.as well as the master cylinder. was this stuff for what and what year did the parts come from or were they ordered for? were all of these orderd for the axle and truck they came off of?

do you trust that pimpple faced kid on the other side of the phone? and at the parts store?

[Wes]

Wow,
Long read. I didn't see If the rear shoes had been replaced or any mention of drum surface condition. Those can defiantly cause poor braking with heavy
pedal effort. Check that your lining material is not hard/ hardened and your drums haven't been over heated or have hard spots. Some of the asbestos replacement lining materials, are crap. Stick w/ a known brand I have had great results from Bendix.