Hi All,

I'm new to the forum and looking to the experts for some advice.
I have a 93 f350 with a brake issue. Here's the story...
A few weeks ago one of the rear brake lines broke. Since it appears that the brakes lines have been spliced together in other areas by the previous owner, I decided to replace all lines from the master cylinder back. I replaced all lines, hoses and front brake calipers and bled the system starting with the wheel furthest from the master. When the truck is off the brake pedal is solid and remains solid even after a few days. When the truck is running and I press the brake, the truck does stop, but the pedal continues to go to the floor if I continue to apply pressure. I did check for leaks and did not find any and since the pedal is firm with the truck off makes me believe that I don't have a leak. While doing some research on the net it seems that the master cylinder may be bad. When I disconnected the lines at the master to replace the lines, I did not remove and bench bleed the master - should I have? Or should replacing the lines and bleeding the system have been enough? Also, if the master was bad, would the pedal go to the floor when the truck is not running - because it doesn't. It looks like the master cylinder is about $50 bucks which isn't bad so I'm thinking about replacing that next unless any of you have an idea of what the issue may be. Any help is appreciated. Thanks for your time.

Mark

 

Does it stop good?

 

Did you bleed rears first, RABs valve then fronts.......that is the way.......

This is some info... but may not be your problem. If you press the brake and stop, do NOT press HARD continuously once stopped just retain it there with sufficient pressure.

Now some members have replaced springs etc in the RABs valve with better results and others replaced the valve complete......$$$$$$ sometimes with same results.

The Fading Bronco Brake Pedal

 

The truck does stop good. We have some snow here and I can lock up the fronts and the backs will lock for a quick second before the ABS kicks in. I haven't had it out on a long drive yet, but I'm concerned that if I stop at a light or something for a longer period of time that the pedal will slowly go to the floor and I'll loose my brakes and have to pump them back up especially if I'm on a hill, which we have a good amount of here in CT. I never noticed this before replacing the lines, but never paid too much attention to it.

When I bled the brakes, I think I did the RABs last. I'll try bleeding the RABs after bleeding the rears and before the fronts.

Thanks.

 

Sounds, from your description, that you still have air in the system. Bleed as advice from PLC7.3. I know from experience that it can be a pain so good luck.

 

I'm still having brake issues. Since the master cylinder was $50 bucks I replaced it, bled the system and the the brake pedal sill goes to the floor when the truck is running. Since the rabs unit is a little pricey, today I took a piece of brake line, crimped it and used it to block the rear port of the master cylinder. My thought was that if the brakes feel fine, my problem is in the rear and most likely the rabs unit. Well the brakes went to the floor with the rear port blocked. Since everything in the front is new, my last thought was that the brake booster may be bad. I disconnected the hose to the booster, heard a hiss, started the truck and with the truck running, the brakes were solid. So does this sound like the booster may be bad? Or could there still be air in the system? I gravity bled the system by opening the bleeder, on at a time from the furthest wheel from the master and let it reservoir go down half way, refilled it and let it go down to half a second time. Do you think there still could be more air?

 

Several times I have seen this issue on trucks with the RABS valve.

I am not well versed in RABS since mine never had one.

But it does seem like the RABS was the reason for the sinking pedal on most of them if I remember right.
On the pre RABS trucks, I would say master cylinder.
Yes, I know you replaced it, but I have also installed master cylinders thet the new one was worse than the old one.

Better let some of the guys with RABS comment on this one.

 

First of all it is NOT the booster. Second of all it is unlikely to be the RABS valve unit. It sounds like you still have air in the system. Also, going on memory, I am pretty sure the rear port on the master cylinder goes to the front brakes.
When you fitted the new master cyl. did you bench bleed it? When bleeding the system did you bleed RR then LR then RABS unit then RF then LF? If you bleed them using the brake pedal close the bleed screw each time before the pedal is allowed to come back up.
If you do suspect the RABS valve unit then you can either join the two brake pipes to bypass it or you can unscrew the big hex plug on it (under spring pressure all the way off), remove the spring and replace the plug. Pump the pedal a few times to fill the chamber and see if it helps. A lot of guys bypass the RABS anyway as it is not much use. However if it makes no difference then just replace the spring and the unit is working again.
Hope this helps.