Infamous Sinking Brake Pedal

[Fordfleet]

Ran the problem past the guys at NAPA. They remembered a local fleet maintenance company having a similar problem and they used a '97 master cylinder to solve it, which is a 1 1/4 bore.

So, I tried the same thing. Maybe it's just coincidence, but the brakes work good enough now to pass the state inspection brake test machine.

Now, if I can just figure out why the brake pedal won't return on it's own until the truck warms up in this cold weather.

 

[94turboidi]

It probably didn't change anything its just that the bigger bore has to move more fluid so it can compress or expand whatever was making the pedal to sink before faster. I have one now since I converted to hydroboost.

 

[MidnightBlade]

when you crack the bleeder are you cracking fast or slow?

 

[DaytonaBill]

I had this problem once, the thing is, these brakes have to be pressure bled...

This is what I used... and it worked...

http://store.motiveproducts.com/ford-three-prong-bleeder--0107-p38.aspx

 

[oldmisterbill]

You either have a leak somewhere or you have air in the system still. If your lines don't leak and you have no air in the system its impossible to have a sinking pedal because your fluid don't compress.

Unless the Rabs valve is bad or the master cyl is bypasing.

 

[adrianspeeder]

Unless the Rabs valve is bad or the master cyl is bypasing.

Both are new.

I had this problem once, the thing is, these brakes have to be pressure bled...

This is what I used... and it worked...

http://store.motiveproducts.com/ford-three-prong-bleeder--0107-p38.aspx

Built one.

when you crack the bleeder are you cracking fast or slow?

I'd say normal speed like I've bleed every other truck I've done.

Adrianspeeder

 

[BlueBreadTruck]

you shouldent *need to power bleed the brakes. I had a front line go on my 94'....replaced that and a new caliper and pedal is fine now after a ton of bleeding to get ALL the air out

 

[adrianspeeder]

I know you "shouldn't". I've done 100+ brake jobs, and never had a problem. Hence the driving me crazy situation of this truck.

Adrianspeeder

 

[alienturtle]

Iv seen defects in "new" parts just an idea

It seems as if everyone has given there ideas of what it could be. Brakes are not rocket science. oil in..air out... You seem to know exactly what to do and are doing it the correct way. And if you back blead the brakes and there are no leaks anywhere then i may be looking at that little percentage of a chance you got a bad "new" part.

 

[The Warden]

Iv seen defects in "new" parts just an idea

It seems as if everyone has given there ideas of what it could be. Brakes are not rocket science. oil in..air out... You seem to know exactly what to do and are doing it the correct way. And if you back blead the brakes and there are no leaks anywhere then i may be looking at that little percentage of a chance you got a bad "new" part.

I was about to say the same thing in regards to the master cylinder, except if I'm reading the latest posts properly, it looks like it's already been replaced a second time.

Have you looked closely at the rubber hoses going to the calipers to make sure they're not giving way at all? I know they've been replaced, but could still be an issue.

For that matter, are you 100% certain the calipers are working properly without any sort of internal bypass?

EVERY time I've ever dealt with a sinking pedal, it was a bad master cylinder, so I'm at a loss here

 

[icanfixall]

Your rear shoes are probably out of adjustment. To check for this problem apply the parking brake. If the peddle sets near the floor then the rear brakes are out of adjustment. The self adjusters really don't function as they should... Never have as far as my rig goes. When you brake for a stop and then feel the brake peddle slowly sink to the floor whats happening is the brake fluid is continuing to fill the wheels cylinders in the back. So the peddle sinks. Hand adjust the back shoes and test drive... I'll bet you will have better brakes and no sinking peddle. I found this out years ago after I ruine several sets of rotors and pads up front.... My truck was stopping on only the fronts... they overheat... Warp and burn out the pads.. Its just a cross we have to bear with these trucks....

 

[alienturtle]

Your rear shoes are probably out of adjustment. To check for this problem apply the parking brake.

i believe he said he plugged the lines going to the rear at the master cylinder and he still has a problem

 

[BigRigTech]

Bypass the rabs valve temporarily and see what happens...I had this exact issue with my 92, a new master cylinder didn't help....Deleting the rabs valve cured it - lots of brakes now....My only concern is to see how it acts in the snow when I'm plowing - I will have to wait and see if it locks up too easily in the rear.

 

[zrexryder]

Sounds like your master cylinder is is ******* by the front into your rear port, have someone push your brakes while you look in the reservoir to see if it rises.

 

[Cincinnati Guy]

When you press the brake pedal should you see any bubbles or movement in the master cylinder resivor?