F250 to F450 master cylinder

[Jeff Dodson]

tell if its OK or if it will fit, to put an F450 master cylinder on my F250. This is what I'm told to do to fix the bleed down problem. I pulled my boat the other day and had to jam on the brakes. Well when I did, the brakes smoked as if they had locked up like before. I replaced both front calipers previously because they locked up. They did not lock this time, but I sure dont want another close call pulling my boat or my camper for that matter. Please HELP

 

[icanfixall]

Yes, it will fit but some rat file work will need to be done on the mounting holes. First thing you really need to do is make sure your rear brakes are adjusted up. They need to be adjusted by hand because the auto adjusters just don't work. Does your emergancy brake engage 1/2 way or more down? If so adjust up the rear and you probably have better brakes. When the rears are out of adjustment all the braking is done by the fronts and they will overheat and crack the rotors. Mine did 3 times before I found out what was wrong. Your brake peddle will probably be very low also. When mine gets down half way to the floor I adjust the backs and all is well again.

 

[Mr_Roboto]

I will second "adjusting your rear brakes". My wrecker is horrible with the back brakes, and all the self adjuster parts are there and working. I still have to get under there and manually adjust them if I want to stop.

Tighten the adjuster till the brake just drags, then use a small screwdriver to hold the ratchet arm back and brake off the adjuster slightly.

 

[oldmisterbill]

On a F250 I would really question the gain by putting on a different (other then origional style)master cylinder??? F250 brakes always seemed good to me.What is the purpose or gain? What year is your truck? I think many master cylinders have been changed due to a faulty RABS valve.( I changed 3 on my F350 B4 I found the real problem.My brakes finally got so bad that after bleeding them to death my brakes would only stay up fpr a couple hundred miles then go back to excessive bleed down.With little or no brakes)

 

[Jeff Dodson]

well the mechanic that checked my brakes said the RABS valve was good and he also adjusted the rear brakes and they still bleed down. He also said that he had received a service builitin (sp) from Ford that the repair was to change the master cylinder to one from an F450, also saying that the holes needed to be wollared out to fit. Then that would make my nrakes hard like they are suppose to be. anybody know anymore about this , agree, disagree, I'm open for suggestions, I know these brakes aint right, and I really need them to be

 

[towcat]

The RABS valve goes into bypass mode and you will have the sinking feeling.
btw. the F450 master is about a 1/4" wider than most f250's and f350's. so do your homework first to see if you will have this problem.

 

[oldmisterbill]

One thing we need to remember is that if we sit still and stand on the brakes with the engine running(so we have good vacume) the pedal will slowly sink.(very slowly)Yet if we drive it normally the brakes will be more than adequate,even sitting still on a steep hill!On the other hand if the bleed down occurs when a hard stop is made(or during normal driving)you have a problem.I only know that when I first got my F350 the brakes were fine for pulling my 30Ft gooseneck(I felt very secure with very heavy loads).When all my brake problems occured it was scary to say the least to drive it with a trailer or just empty(good thing my trailer brakes were in good condition).Now with out the rabs valve I can stop fast enough(even with a heavy load) to get myself into troubleif I don't pay attention. Again my feeling is maybe there have been to many changes in master cylinder sizes etc.when we really needed new Rabs valves.

 

[sassyrel]

some have also said--that the booster may be starting to get bad--i/e, that it is leaking enough vacuum at idle--that it goes down--as your vac pump doesnt create a lot of vacuum at idle--shut truck off--push brake pedal about 4 full times--vacuum should be gone--then---push on brake pedal fairly firm--and brakes should stay at whatever height--if they start to go down--would say master cyl is bleeding by---internally---

 

[MIDNIGHT RIDER]

What, and where, is this RABS valve??

Did I understand correctly that some have removed it, and gotten better brakes with it gone??

What part does it play??

Also, I thought I would replace my brake-master with the SuperDuty master.

When the guy at the parts desk put them side-by-side, they were identical, with the exception that the F-350 part # comes with the reservoir, and the F-450 is the cylinder only.

The bores miked the same, they were identical.

Am I missing something??

What year/model master-cylinder is the one that will swap, and be an improvement??

Also, I have learned that wheel-cylinders for a 1978 GM ton, with Corporation rear-end, are a direct bolt-in swap for the wheel-cylinders in both Dana 60 and 70, with the exception of the brake-line sticking straight out, instead of angling forward.

The bore of the GM cylinders is quite a bit larger, probably part of the reason that a GM truck will stand on it's nose, whereas a Ford, or Dodge, will run through the bridle.

 

[oldmisterbill]

I would not rccomend removing the RABS valve if that is the problem.REPLACE IT!! As I said B4 I have a 30ft gooseneck trailer it is fine with it loaded,empty a panic stop could be a problem( REAR WHEEL LOCKUP!) Empty it is trecherous on anything but a dry road.It will start to be used as a daily driver for a while so I am going to put in a RABS valve.Why change a good thing Just maintain it.

 

[Mont91]

This is the second time to try and post today.

Dodson, have you tested or replaced the master cylinder(MC) yet?

If you haven't, test it this way. Remove the brake lines, screw in plugs. If the pedal still sinks the MC is bad, if it does not the MC is good.

I stronly disagree with the mechanic about the 450 MC.

Sinking pedal is most likely {A} bad MC {B} improperly adjusted rear brakes {C} bad Rear Antilock Brake System (RABS) valve.

The only TSB I know of says basical to do the following.
Check the MC, then adjust the rear shoes until they rub fairly hard but so that you can still turn the wheel, if the pedal stil sinks it is probably the RABS valve. I have never heard of replacing the MC with a 450 one. Ask him for the TSB number. According to Ford there is no way to "test" the RABS valve.

Many mechanics do not realize how critical rear brake adjustment is. They are used to "just get it close and it will work" that does not work here. Even a slightly out of adjustment rear brake makes a noticable differance in the brake pedal.

The RABS valve is located in the frame rail under the driver.

A weak vacum pump will cause a sinking pedal, but a differant sinking pedal.