In process of redoing brakes on my 91 D350 dually rear wheel drive. The master cylinder I got from parts store is different from the one on the truck. I have went to total of 3 parts stores and they all only show 1 master cylinder for 84-93 one ton, the same as I got from the 1st parts house.
The master cylinder presently on the truck has the large reservoir on the front (towards truck grill) the opposite of the new one from parts store. Also the fittings on truck don't match size wise to the new one. The present one also has bleeders screws which the replacement doesn't.
Was there any obscure variation in master cylinders or has some one adapted the wrong master cylinder on this truck? The lines look factory with the spring around them and don't look cobbled. Brakes had worked well for a long time.

 

Go to the dealership parts department and have them look it up. Get the price, part number and availability but don't buy it. Then go to NAPA with that part number and have them cross reference for their part number. They may even have a picture for you to ID. If NAPA can't cross the part number it may be a dealer item only.

 

Thanks for help! Sorry to be slow with update. Did as you suggested and went to dealer. Ultimately found out some one in past installed WRONG master cylinder. Upon closer inspection even noted that they had drilled a hole through the divider so the fluid could equalize from the large reservoir to the small!
Purchased rebuilt master cylinder. Still do not have good brakes. Some times if I hold pressure on brake pedal it still fades away as old master cylinder was doing. Is there anything other than a bad master cylinder that can cause this fade away problem?
Intend to go through front brakes as well, but that will have to wait till warmer weather now.

 

Have you looked for leaks at the slave cylinders?

 

How's your brake booster?

 

Wheel cylinders on rear are new. Brake booster is new reman. No fluid leaks.

 

Did you bench bleed the master cylinder and then thoroughly bleed all the wheel cylinders/calipers. Check all your lines and rubber hoses for any leaks. Also, I have gotten rebuilt master cylinders before that is bad out of the box (just saying).