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1978 W200 Brake issue?

4K views 35 replies 8 participants last post by  dodge82273 
#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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#2 ·
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.
 
#8 ·
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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#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.
 
#5 ·
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
 
#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.
 
#9 ·
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.
 
#10 ·
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
 
#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.
 
#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.
 
#14 ·
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 :)
 
#15 ·
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.
 
#16 ·
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.
 
#17 ·
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 ...
 
#19 ·
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
 
#20 ·
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.
 
#22 ·
Howdy
I measured the pushrod length very carefully when I put the new MC on it one. I later saw a cool simple tool that did the same thing. The master cylinder, booster, and proportioning valve that are on the truck now, all came off a truck that was ( bad motor) running and driving just a few weeks ago.
 
#23 ·
any clearance between the pedal rod and the booster ? I'm just putting potential "problems" out there because it cannot be a mysterious thing . My experiences came back to bad masters , and stuck prop valves , but never has a "making a turn " problem unless the wheel bearings were loose as hell .
 
#24 ·
So update today and progress. I got help and bled the brakes again, no air in the lines. Trucks been sitting at least a week, no leaks, drips or loss of fluid. The e-brake pedal in the cab has been rusted up for a long time before I got the truck. So I replaced the pedal and cable inside the cab with a good clean working pedal off the parts truck. Hooked that all up, tightened that up till the slack was out. Pressed the pedal and it goes about half way down and gets tight. I had the rear end off the ground already because I was going to tighten up the rear shoes. I grab the driver tire and it spins. The passenger doesn't. I had adjusted the rear shoes before and thought they were close, but obviously not. So some more adjusting, finally enough brake to get it on the road to actually bed the brakes with some hard stops, and a drive up my hill and the brakes work, they dropped a little but caught well before the bottom. I'm thinking someone went in and just fixed the passenger side rear brake sometime in the past, the wheel cylinder was good on that side and I bet they replaced the drum. The driver side wheel cylinder was bad and I bet that's the original drum. I'm guessing my hill is rough enough to to shake slack in shoes when I drove up it since the shoes were so far out of adjustment to start with. Pedal feels 100% better like it should. I'm going to adjust the driver side a little more tomorrow and I think it should be good. Thoughts?
 
#25 ·
look close at the arm that operates the p brake inside the drum , they rust up , stick , and cause the wheel cylinder to have a "gap" between it and the shoes , makes the system act like the rear shoes are WAY loose out of adjustment even tho the shoes/drum are touching because that arm is holding the shoes out ......
 
#29 ·
the spring OVER the p cable INSIDE the drum is what "returns" the p brake ARM in the drum , with cable loose it should return that arm freely .
 
#30 ·
well, after a final adjustment and test driver, its worse. Real good brakes and then pedal just goes to the floor with no rhyme or reason.
3 different master cylinder, 2 new
2 different brake boosters , 1 new
3 different proportioning valves
all new soft lines
new pads, shoes, and wheel cylinder
no leaks, no fluid loss.
 
#31 ·
O.K. pedal goes to floor without fluid loss means there is no fluid movement/ air or bypass by master . so , well the combo valve is SUPPOSE to stop flow to either the front OR rear when a line blows on that end allowing a LOW but serviceable pedal . ( brakes on good end only ) TO the floor means one of 2 things : either the combo valve was not centered , or is stuck . OR the master did not return all the way and has NOT pushed any fluid or it has defective cup seals .. when it does get a GOOD pedal , place both feet on pedal , press down hard , this is SUPPOSE to "center" the combo valve .

next option , after all this work , do you , can you , make steel brake lines ? some do / can some have never so I ask . If yes , what I would do is get a combo valve from a slightly newer truck , ( 92 up ) its aluminum and mounted on a bracket bolted on where the master is . make up the new lines , replace the steel combo valve thing under your feet , same function . Bleed the master , then gravity bleed the lines open 1 at a time , l/front .... this SHOULD keep the valve "centered" .

you are using 12 inch shoes in a 12 inch drum not 11's ? no offense , but I / we don't know what you know or don't know LOL

I suspect the master is not bled or not returning , or the combo valves on the rail suck . YOU are NOT alone , I have replaced 3 masters on 1 try to fix brakes "pulling my hair out " it was a bleeding problem , one end not bled enough , pedal OK then combo valve shifts , pedal bad . try to bleed BUT combo valve shifted can't bleed on end properly .. years later another truck bad combo valve drove me nuts even had a bleeder ball ( pressure bleed ) on that one ... take a deep breath and continue :)
 
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#32 ·
Update

The spring is over the parking brake cable, or at least it was in the pic I took before I pulled everything apart :) I am pretty good at putting things back together like they are in the pictures. Doesn't mean a previous owner didn't mess up though.

They are 12 inch shoes, didn't throw the old ones away yet.

So I started with the easiest, pushed the pedal real hard and started re-bleeding the brakes. Passenger rear went fine, no air. Driver rear pedal didn't feel the same, no air, so I stopped. Thinking it tripped the combo valve. Pushed the pedal real hard a few times and test drove. Same issue, random pedal to the floor.

Split the master and the booster, measured and there was a bit of a gap. Adjusted that and of course a little bit for good measure. Put the master back up, it seemed to fit flush with no issues (wrong), Bleed the master cylinder, hooked the lines up and gravity bled that.

Side note here, I was always under the impression that on the older heavy cast master cylinders, the front, larger chamber goes to the front brakes. Well on the 83 aluminum master cylinder with a plastic reservoir, the back line goes to the front brakes. Found that out when I swapped the master and combo valve.

So test drover the truck all over the place and on my hill several times, Great brakes never lost pressure Actually too good of a pedal. Seems the front brakes are not releasing all the way and dragging. So when thing cool down this evening I will split the master cylinder again and put some slack back in the brake rod.

Its a change and a easy fix and test drive. See what happens then.
 
#33 ·
some slack needs to be there ....1/32 or more
 
#34 ·
Well, several test runs and small trips around the house. Do a decent test drive on some rural roads. No issues , great pedal. Go to town and back out of a parking lot and the pedal goes to the floor, AGAIN. :mad::mad::mad::mad::mad::mad::mad::mad:
 
#35 ·
That's how the brakes went on my 95 F350. Good 98% of the time. Once in a blue moon - pedal hit the floor and no stopping.

I upgraded the booster and the problem never resurfaced.
 
#36 ·
the one thing I haven't mentioned . I experienced pedal to floor on a overloaded (17000 pounds) new 1984 E350 when confronted with several stops in a short time/distance , cause brake fluid boiling in the front calipers , cure dot 5 fluid change.
 
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