B pedal slowly going to floor w/eng running

xt01815

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Hey, um I'm new ish to the site. Hope im posting in the right spot. So my first post is about trying to get ideas on other places to look and check. My rig is an 88 f350 quad cab 5-speed 2wd dually. So my problem is eng running hold the b pedal and it slowly sinks to the floor. My first thought was master. Second was booster. Third was pin hole some where. And finally to double check all fittings and bleed the system. This all started cause my clutch went out and my inspection is dead. So knowing I haven't done the rear brakes.........I did them. Along with a mass of other normal maintenance that I did, when it came time for the brakes I did the following.

Rear shoes (all)
Rear drum hardware (all)
Wheel cylinder (all)
Front pads (all)
Master cylinder (new)
Booster (reman)
Full Rear line replacement from master to both rear wheel cyl (w/correct fittings)

So I put the new master, booster, and lines in only through different stages of massive aggrevation trying to guess where my air is coming in from. I've manually bleed. Used a power bleeder. Bench bleed. Gravity bleed. You name it I bleed the crap out of it. So I keep getting suspicions of the new driver rear wheel cyl cause I keep getting air from it. Then I took the time to replace all the rear line system (cause it's old anyway what's it gonna hurt). So then I power bleed it again. Now I'm getting air from the new pass wheel cylinder. I'm wondering if I should bypass the proportioning valve under the fire wall. If so would it be OK to leave it bypassed. And I think I already know this but just in case, is it possible to have an air leak in the front brake system but be able to pull it to the rear system with all of the bleeding procedures I mentioned above minus the gravity one lol. Only reason I ask is cause they are both connected via master. And thought maybe my old and new ones could have a bad spot allowing for it. Idk just thinking to much I guess.

So what are your thoughts/ideas. Maybe someone has had a similar issue with all the new parts.
 

Knuckledragger

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Does the pedal pump up? Seems like you still need to bleed more. If bubbles come out at all, more bleeding is necessary. Unfamiliar with power bleeding, never had a problem with the old fashioned way.
 

xt01815

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Yes pedal pumps up at first. And I'm on my 4th LARGE bottle of fluid if that means any thing on how much I've bleed so far.
 

Knuckledragger

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You keep adding fluid? Finding no leaks? That usually means either: bad master seals (would not use lots of fluid) or bad power booster (it happens). If you continue to bleed and keep getting bubbles, there is a leak. Possibly in portioning valve, possibly elsewhere. Where is all the fluid going?
 

danda

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vacuum line leak? you can check it with a vacuum guage.

I had a similar system where brakes would pump up, then go to floor. I bled the crap out of it. finally, I checked vacuum, saw it would drop rapidly, replaced hose, and voila! brakes.
 

xt01815

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Its the air that Im worried about. I keep pulling from somewhere else. The old master that I thought was bad was doing the same thing. So that eliminated my reasoning for thinking the new one is bad. Same for the booster. I checked the air pump, then check valve on the old. And check again on new. It too acts the same. So back to my original q's and the end of my original post. If I bypass the proportioning valve and if it turns out that's my source of problems will I be able to be safe driving down the road like that.
 

xt01815

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Danda
Which vac lines would you be talking about. Cause other than cruise, booster and the old coffee can I'm not sure where else
 

Big Bart

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Hey, um I'm new ish to the site. Hope im posting in the right spot. So my first post is about trying to get ideas on other places to look and check. My rig is an 88 f350 quad cab 5-speed 2wd dually. So my problem is eng running hold the b pedal and it slowly sinks to the floor. My first thought was master. Second was booster. Third was pin hole some where. And finally to double check all fittings and bleed the system. This all started cause my clutch went out and my inspection is dead. So knowing I haven't done the rear brakes.........I did them. Along with a mass of other normal maintenance that I did, when it came time for the brakes I did the following.

Rear shoes (all)
Rear drum hardware (all)
Wheel cylinder (all)
Front pads (all)
Master cylinder (new)
Booster (reman)
Full Rear line replacement from master to both rear wheel cyl (w/correct fittings)

So I put the new master, booster, and lines in only through different stages of massive aggrevation trying to guess where my air is coming in from. I've manually bleed. Used a power bleeder. Bench bleed. Gravity bleed. You name it I bleed the crap out of it. So I keep getting suspicions of the new driver rear wheel cyl cause I keep getting air from it. Then I took the time to replace all the rear line system (cause it's old anyway what's it gonna hurt). So then I power bleed it again. Now I'm getting air from the new pass wheel cylinder. I'm wondering if I should bypass the proportioning valve under the fire wall. If so would it be OK to leave it bypassed. And I think I already know this but just in case, is it possible to have an air leak in the front brake system but be able to pull it to the rear system with all of the bleeding procedures I mentioned above minus the gravity one lol. Only reason I ask is cause they are both connected via master. And thought maybe my old and new ones could have a bad spot allowing for it. Idk just thinking to much I guess.

So what are your thoughts/ideas. Maybe someone has had a similar issue with all the new parts.
X,

Welcome to oil burners!

Your booster is not going to add air bubbles to the brake system. So no need to chase that.

Sounds like you still have air in your lines or you have a air leak somewhere.

1) Some have had challenges getting the ABS part under the cab to bleed with the truck brakes. Not the proportioning valve, I think it is called a rabs or ravs valve. I will let others weigh in on this.
2) I would check for a loose brake line fitting or damaged/bad brake line flare. (Master cylinder, your néw brake line, wheel cylinders etc.) Also I had issues with the part that goes between the master and the one brake line sealing. I used a sealant that is oil resistant to get the threads to seal to the master.
3) You likely know, but it helps if bleeding the rear brakes to Jack up the rear of the truck. When doing the front jack up the front. It puts an angle on any long brake lines to help the brakes bleed. Start with passenger rear, driver rear, passenger front, driver front.
4) I do not recall how the system is plumbed, other can chime in if front and rear circuits are isolated.
5) In today’s world do not assume any part is good out of the box. You may want to have the master warrantied if you cannot find another leak. But agree it would be odd two masters are both leaking in air. Maybe you are right about the wheel cylinders but check the brake lines to them to make sure they are tight.

Let us know what you find next.
 
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xt01815

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So this morning I did the following...
-Put teflon tape on all three bleeder valves. Reason was cause I noticed that the moment I slightly opened them they wiggled a bit while bleeding. So I then noticed after that I wasn't pulling in so much air.
-Power bleed the system.
Gnathv starting from PR. I had to wait about 10-15 min before I got solid fluid. Went to DR. Got solid fluid immediately. Then to rabs again. Got solid fluid immediately also.
-Started engine
-pumped brake a couple of times
-Held brake
Still sinks to the floor.
I guess I'll check the vac lines, maybe cut the ends off to give a fresh end just incase. Maybe disconnect all rear line fittings and put Teflon tape on to cover my p's & q's
 

Big Bart

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So this morning I did the following...
-Put teflon tape on all three bleeder valves. Reason was cause I noticed that the moment I slightly opened them they wiggled a bit while bleeding. So I then noticed after that I wasn't pulling in so much air.
-Power bleed the system.
Gnathv starting from PR. I had to wait about 10-15 min before I got solid fluid. Went to DR. Got solid fluid immediately. Then to rabs again. Got solid fluid immediately also.
-Started engine
-pumped brake a couple of times
-Held brake
Still sinks to the floor.
I guess I'll check the vac lines, maybe cut the ends off to give a fresh end just incase. Maybe disconnect all rear line fittings and put Teflon tape on to cover my p's & q's
That sounds like your master cylinder is defective. Your brakes would maybe go 1/3 Tom 2/3rds down to the floor and feel spongy but not go to the floor if you had a little air in your lines.(Guessing you did not have any.).

I would do this test. Bleed again, then just hold the pedal down, see if it sinks. When you pump it may be sucking air into a line. But if you just push and hold and it goes to the floor it’s your master cylinder. Maybe release slowly once or twice, then see if it goes to the floor. Either way at that point I would warranty the master cylinder.

One last thought. There is a rod that goes from the booster to the master cylinder. Maybe confirm the old and new are the same length and have the same travel.
 

Big Bart

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So this morning I did the following...
-Put teflon tape on all three bleeder valves. Reason was cause I noticed that the moment I slightly opened them they wiggled a bit while bleeding. So I then noticed after that I wasn't pulling in so much air.
-Power bleed the system.
Gnathv starting from PR. I had to wait about 10-15 min before I got solid fluid. Went to DR. Got solid fluid immediately. Then to rabs again. Got solid fluid immediately also.
-Started engine
-pumped brake a couple of times
-Held brake
Still sinks to the floor.
I guess I'll check the vac lines, maybe cut the ends off to give a fresh end just incase. Maybe disconnect all rear line fittings and put Teflon tape on to cover my p's & q's
Vacuum lines are not your issue, they assist the brake booster but do not cause the pedal to go to the floor.

Do not put Teflon tape on your brake line flare joints. If they leak replace that line. The flare joint should seal against the male connection on the other side. The nut pushes the two together to seal them together.
 

danda

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Which vac lines would you be talking about. Cause other than cruise, booster and the old coffee can I'm not sure where else
a leak in any line would cause vacuum loss. In my case it was the hose between the vacuum manifold and the brake booster.

So the symptom I was seeing was that with the truck off I could pump the brake up and it would stay firm-ish. Not super stiff, but would def not go to floor.

As soon as I started the truck, it would fade/drop quickly. I could still pump it back up, but it fades right away after.

If you have these symptoms, vacuum might be your issue. But if its going to floor with truck off, it's most likely air or a fluid leak somewhere.

edit: oh, and in the course of digging into this, I determined that my low vacuum sensor is not working either. If it were, the cause would've been obvious from the start.
 

xt01815

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I spent the rest of the day inspecting and bleeding again including the front which now im getting air there too. Vacuum seems fine. No visual leaks under. I must have blown my garage circuit 7 times from the air compressor running for so long while using the power bleeder cause I'm by my self. Ugh it sure is hard getting from under my truck running to the basement and back again.
 
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