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Discussion Starter · #1 ·
Hello

Have a 78 W200 440/727/NP203 truck.

The brake issue is without warning the pedal will go to the floor and you have no brakes!
Previous owner put on a new master cylinder and brake booster, that didn't fix it.

I put on a new master cylinder( figured it was a bad "new" master cylinder, new check valve in the booster, and all new soft lines. That didn't fix it.

So. New front pads, ( the old ones were fine), cleaned and checked everything that moved on the front, made sure the pucks moved, made sure all the fittings were tight.
New rear shoes (they were iffy), new wheel cylinders(one was bad), cleaned everything, checked all the fittings.

Took a working master cylinder, booster, and proportioning valve, off a 83 W250 with a cracked block, gravity bled everything, then bled all four corners again using tubing and a bottle. Drove the truck around the property for 15 minutes, brakes seemed low in the travel and soft, but nothing was bedded in. All fine, drive up a slight rise and turn into my shop and pedal goes to the floor! No leaks anywhere and does not use brake fluid. I am at my wits end and none of my local truck friends have ever herd of an issue like this.

If anyone has any suggestions I am all ears. Thanks.

Things I am going to try when I get time,
Unhook and plug the vacuum line to the booster and test drive it.
Get an emergency brake pedal from a donor truck and replace the one in the cab that is frozen. Adjust emergency brakes.
 

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Thats interesting.

Is the brake light on the dash coming on? When the pedal goes to the floor, did you try pumping the brakes?

The one thing I do not see mentioned is, was the master cylinder bench bled before it they were installed? Seems odd that two people would miss that step, but I have to ask.

Everything is mechanical from the pedal, to the master. They would be either broke, or not. From the master down, it is all hydraulic, there would have to be a leak to cause the pedal to go to the floor. Rear brakes way out of adjustment, could cause the pedal to floor, but it would do it all the time. The proportioning valve, would have to have a bad leak, which would be easy to spot.

That leads me back to the master. I would try bench bleeding the one you have, and then re-bleed all the lines.

Were the replacement masters new, or rebuilt. I had two bad rebuilt ones in a row, so now I only get new ones. So do not rule out one being bad.
 

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Discussion Starter · #3 ·
SB,

No brake light on the dash
The previous owner said he bench bled the 1st master cylinder. I bled the second new one on the truck. Both were new Raybestos from RockAuto. The 3rd one on there from the 83 was working fine just a few weeks ago,, so I just gravity bled the whole system after that.

Pumping the brakes will get some pressure back, then the brakes act normal for a while, then to the floor they go.
 

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your combi/proportioning valve is NOT holding the rear brake shoe pressure , the valve is designed to hold slight pressure on the rear brakes , if you loose that pressure the pedal go's to the floor
 
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your combi/proportioning valve is NOT holding the rear brake shoe pressure , the valve is designed to hold slight pressure on the rear brakes , if you loose that pressure the pedal go's to the floor


View attachment 617752

I agree with dodgeboys, chased my tail on my 83 a few years back, replaced everything but the prop valve and be damned that was it (last thing I changed).

I would also check adjustment on brake shoes in back. As you already know make sure master is full of fluid and rebleed after any changes/adjustments.

Neil
 

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Discussion Starter · #6 ·
Well a new proportioning vale it is. When I install it I will re-blead the master cylinder, and then of course re-blead the system.

Thanks
 

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Discussion Starter · #7 ·
So back again. Put on the 3rd proportioning valve, It was out of a running driving parts truck, it worked. Bled the master cylinder, bled the brake system, had a hard time getting the rear brake line to seal on the proportioning valve this time. Drove it all over the property, everything works, but pedal is low in the travel. Some hard stops, can lock up the front, but not the rear ( didn't readjust rear shoes yet). Final test, I drive up the little incline to my shop. Its maybe 150 feet at 15 degree. Turn to come in to the shop and no brakes, pedal all the way to the floor. No leaks, no drips, nothing is damp, I'm at a loss.
 

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That leads me back to the master. I would try bench bleeding the one you have, and then re-bleed all the lines.
Do the above, and then bleed the lines by having someone push on the pedal, You want to get the fluid flowing through the lines fast.

I still think you still have a bad master.
 
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Adjust the rear shoes?

My pedal was really low the other day. I added a return spring to the parking brake cables, and adjusted shoes. Pedal was much improved. Go figure.
 

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on ANY front disc rear drum brake system , the rear brakes control the brake pedal height and feel , so your either NOT bleeding it right or like George says the master is bad , me l,m thinking you have the rear rubber line ballooning , l,ve seen it happen a few times but its rare
 

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Discussion Starter · #11 ·
SB I bled the master cylinder on the truck again with one of those kits with the screw in plastic inserts. This will will be the 4th master cylinder on the truck, one on the truck with previous owner, new one the previous owner put on, new one I put on, this used one that was good at least 3 weeks ago. I gravity bled the whole system, and then bled the system by myself using clear tube and a bottle. I thought swapping emergency brake pedals was going to be simple, but that little clip that holds the cable in the pedal assembly is a pain. I will round up someone to help bleed them again and then I am done for a couple of days till I get more time to mess with it.
 

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Discussion Starter · #12 ·
DB, due to reading this forum and others, when I replaced the master cylinder the 1st time, I got all new soft lines. The soft lines are from Rock Auto, pretty sure all brake parts the 1st time were Raybestos brand, still had china on the box though.
 

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definitly take a break , you fuk with something too much and you,ll get sick of it and burn it to the ground , which you dont wanna do , step back and take a breath
 
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Discussion Starter · #14 ·
definitly take a break , you fuk with something too much and you,ll get sick of it and burn it to the ground , which you dont wanna do , step back and take a breath
I'm there now, even if my technique wasn't perfect I shouldn't have the pedal just going to floor every time! I may have time this evening to bleed it with help and then its sitting for several days. Thank God its not my daily driver :)
 

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Here's a long shot but I've seen it before. Your description of when this happens included the phrase "turn into shop" a couple times. Is it possible that the calipers are hitting something and compressing the pistons?


I ran into this years back on a friend's Coronet. He had done a disc brake swap and on tight turns the caliper hoses would hit the shocks and push the calipers back just enough to make the pedal really soft. Turns out he had the wrong calipers and hoses plus the spindles were on the wrong sides.

Maybe just sloppy front bearings too.
 

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Here's a long shot but I've seen it before. Your description of when this happens included the phrase "turn into shop" a couple times. Is it possible that the calipers are hitting something and compressing the pistons?


I ran into this years back on a friend's Coronet. He had done a disc brake swap and on tight turns the caliper hoses would hit the shocks and push the calipers back just enough to make the pedal really soft. Turns out he had the wrong calipers and hoses plus the spindles were on the wrong sides.

Maybe just sloppy front bearings too.
Good catch, bad front wheel bearings can do that.
 

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question , is there any free play between the master's piston and the trucks push rod ? if the master does NOT return ALL the way it will never bleed on the truck . If the caliper/s are NOT floating on their slides due to wear , the fronts will consume too much fluid before moving the pistons and cause a low pedal .. a bad combo valve is my first guess tho . good squirt from one end ( front/rear "ends" ) but not the other ? bad valve . newer ones that hang off the master will retro fit with some new lines made up ...
 

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Discussion Starter · #18 ·
Here's a long shot but I've seen it before. Your description of when this happens included the phrase "turn into shop" a couple times. Is it possible that the calipers are hitting something and compressing the pistons?


I ran into this years back on a friend's Coronet. He had done a disc brake swap and on tight turns the caliper hoses would hit the shocks and push the calipers back just enough to make the pedal really soft. Turns out he had the wrong calipers and hoses plus the spindles were on the wrong sides.

Maybe just sloppy front bearings too.
All stock front end and the bearings are tight. I put new pads on the front and cleaned everything. I didn't see anything unusual, but will double check. I will do so some turns on flat ground to the right and see if I can duplicate the issue.
 

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could you have one of the front caliper hoses "twisted' and holding pressure till you turn the wheel and it leases the pressure and the pedal drops / the diagrams are NOT of a dodge but to show what l,m talking about
617988


617989


617990
 

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I'm with dodge82273. I was going to say to look at the pushrod and m/c piston and how they relate to each other. The m/c pushrod might be too short. You should always use the same pushrod/master cylinder combo from the trucks you pillage from. Not all are the same.
 
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