Brake problem - HELP PLEASE!!
Post Fiend
Joined: Jan 2005
Posts: 8,402
Likes: 10
You are quite right in what you say about how the master cylinder should behave with both ports blocked. Yours is not doing that so, unless the actuating rod is the wrong length, you have a bad master cylinder.
One thing for sure it is not the booster.
One thing for sure it is not the booster.
Thread Starter
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VT Chapter Ldr
Joined: Apr 2004
Posts: 96,076
Likes: 294
From: Orange, Vermont
is there a way I can check the length to be sure and what is the length. When I put the new booster on, it described a way to do it but I think I sent the directions back with the old booster. Talked about pushing the brake pedal and looking for bubbles or something like that.
Senior User
Joined: Jan 2009
Posts: 219
Likes: 0
From: Maricopa, AZ
Does the parts store still have your booster core? If so, then you could do a side-by-side comparison, or have them measure how far the actuating rod sticks out, and compare that with what's installed.
Post Fiend
Joined: Jan 2005
Posts: 8,402
Likes: 10
is there a way I can check the length to be sure and what is the length. When I put the new booster on, it described a way to do it but I think I sent the directions back with the old booster. Talked about pushing the brake pedal and looking for bubbles or something like that.
Try this, back off the nuts holding the master cylinder to the booster until you get about 1/8in gap, make sure you back them off evenly. Now try the brakes and see if there is any difference. If there is then you know you have to shorten the rod.
New User
Joined: Nov 2003
Posts: 23
Likes: 0
From: Richardson, Tx
Had a problem kind of like this on my93f250HD. (Pedal going to floor slowly)
Pretty scary and unpredictable.
Replacing the RABS speed sensor in the rear axle (pumpkin) solved the problem.
Have you replaced that sensor?
Pretty scary and unpredictable.
Replacing the RABS speed sensor in the rear axle (pumpkin) solved the problem.
Have you replaced that sensor?
Thread Starter
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VT Chapter Ldr
Joined: Apr 2004
Posts: 96,076
Likes: 294
From: Orange, Vermont
No I haven't tried that but right now I have the rabs valve completely bypassed so it is not in the system at all.
I took it for a little drive yesterday and when I first brake, feels like good power assist, if i let up and brake immediately agian it feels more like a manual system and if I do it agian immediately it feels like I need to push the firewall to the core support to give it enough pressure to brake. also after the first braking the brake light will come on, and then go off after maybey 15 -20 seconds and then when I brake it feels like good power assist. This is a replacement booster I just put on and on my second master cylinder, which I saw was leaking fluid between the booster and master cylinder, so this one looks like it is leaking also. But with it dead headed, if it drops it is now where as fast as with the lines hooked up.
I took it for a little drive yesterday and when I first brake, feels like good power assist, if i let up and brake immediately agian it feels more like a manual system and if I do it agian immediately it feels like I need to push the firewall to the core support to give it enough pressure to brake. also after the first braking the brake light will come on, and then go off after maybey 15 -20 seconds and then when I brake it feels like good power assist. This is a replacement booster I just put on and on my second master cylinder, which I saw was leaking fluid between the booster and master cylinder, so this one looks like it is leaking also. But with it dead headed, if it drops it is now where as fast as with the lines hooked up.
i ain't rite
Joined: Mar 2005
Posts: 65,429
Likes: 5,509
From: Marlboro Mental Hospital.
there are a few things with your "sinking pedal". this is a common occurrence with the diesel trucks.
there are 2 ways to fix it. you can either put a larger 1 5/8 bore master cylinder off of a superduty in it, or convert it to a hydraulic assist unit.
both are discussed regularly in the pre-powerstroke diesel forum.
as for the hard pedal on second brake application thing, either your vacuum pump is going bad, or your booster is bad. my money is on the bad vacuum pump.
there are 2 ways to fix it. you can either put a larger 1 5/8 bore master cylinder off of a superduty in it, or convert it to a hydraulic assist unit.
both are discussed regularly in the pre-powerstroke diesel forum.
as for the hard pedal on second brake application thing, either your vacuum pump is going bad, or your booster is bad. my money is on the bad vacuum pump.
Cargo Master
Joined: Jan 2004
Posts: 3,118
Likes: 149
From: South Dakota
The sinking pedal is a common occurrence on many Ford vehicles of this vintage. When I first read about the sinking pedal on a forum like this several years ago I thought WHAT, so I went out and tried the Ford vehicles I had and to my surprise every one of them had a pedal that would slowly sink if I sat there with the engine running pushing firmly on the brake pedal. I challenge everyone who reads this, try yours.
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Tuned
Joined: Apr 2008
Posts: 406
Likes: 1
From: Cleveland,OH
It has always been my understanding that master cylinder is basically a piston that applies force to the fluid to force it to move through the system to force the pistons and calipers to move. And there should be no pressure loss in the master cylinder. By plugging the ports, or some call it dead heading, if there is no place for the fluid to go, the pedal should move a litte and stop because the fluid can only be physically compressed to a certian point.
i ain't rite
Joined: Mar 2005
Posts: 65,429
Likes: 5,509
From: Marlboro Mental Hospital.
the big difference in master cylinders is the piston bore size. standard master for the F150-F250 is the 1 1/8 bore piston.
there is also a 1 3/8 bore, and a 1 5/8 bore piston master cylinder.
i put the 1 5/8 bore master cylinders on all of my trucks, and the "sinking pedal" went away. plus, as an added side-effect, you got better braking also.
the downfall is you will need to do some modification to the mounting flange to make it fit.
there is also a 1 3/8 bore, and a 1 5/8 bore piston master cylinder.
i put the 1 5/8 bore master cylinders on all of my trucks, and the "sinking pedal" went away. plus, as an added side-effect, you got better braking also.
the downfall is you will need to do some modification to the mounting flange to make it fit.
Thread Starter
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VT Chapter Ldr
Joined: Apr 2004
Posts: 96,076
Likes: 294
From: Orange, Vermont
tjc transport, what is the modification needed and is that from a 96 vintage f350 and it will the inside hole of the booster? Could I put on a f350 booster or do I need modify the mounting on the firewall also? If the master cylinder leaked it may of gotten into the booster also, so I may have to replace it agian.
i ain't rite
Joined: Mar 2005
Posts: 65,429
Likes: 5,509
From: Marlboro Mental Hospital.
i did it on 88 to 92 trucks. the large piston master just fit in the booster hole.
instead of having side to side play, the hole in the booster was exactly the same size as the master itself. i had to clean the hole in the booster completely, and then use the nuts to pull the master into the booster. i had to elongate the master bolt holes to make it fit, because the mounting bolts were wider than the 250 mounts.
instead of having side to side play, the hole in the booster was exactly the same size as the master itself. i had to clean the hole in the booster completely, and then use the nuts to pull the master into the booster. i had to elongate the master bolt holes to make it fit, because the mounting bolts were wider than the 250 mounts.
Thread Starter
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VT Chapter Ldr
Joined: Apr 2004
Posts: 96,076
Likes: 294
From: Orange, Vermont
I bought myself a vacuum gauge yesterday and checked my truck, that was more of a pain in the butt then it should of been because I had to rig up adaptors. I checked it over near the heater box side, then at the booster end of the hose and then at the end of the hose from the pump to the distribution block. Best I could get was 11-12 inches. I am going to see if I can find some else to check it to confirm because like I said I had to rig up some adaptors to fit the different ID hoses, using some stuff from my fuel pressure test kit.
I called the dealer and going through my dad's account it would be $215 for a new pump. Anyone have a less expensive source?
Also last Saturday I drove over to a friends house so he could look at the brake problem and by the time I got there, only like 2.5-3 miles, he noticed brake fluid dripping down from between the master and booster, so it looks like this one has some seapage also. Any way to know if the fluid got into the booster?
I called the dealer and going through my dad's account it would be $215 for a new pump. Anyone have a less expensive source?
Also last Saturday I drove over to a friends house so he could look at the brake problem and by the time I got there, only like 2.5-3 miles, he noticed brake fluid dripping down from between the master and booster, so it looks like this one has some seapage also. Any way to know if the fluid got into the booster?
i ain't rite
Joined: Mar 2005
Posts: 65,429
Likes: 5,509
From: Marlboro Mental Hospital.
i ain't rite
Joined: Mar 2005
Posts: 65,429
Likes: 5,509
From: Marlboro Mental Hospital.
there is also Ed at Parr ford, a site sponsor
Th!nk Parts Guy/FTE Parts Guy
i tried looking up the vacuum pump on his site, but could not find it. but if you want to email him, he will get back to you within 1 day with a price quote.
Th!nk Parts Guy/FTE Parts Guy
i tried looking up the vacuum pump on his site, but could not find it. but if you want to email him, he will get back to you within 1 day with a price quote.