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Brakes locking on after driving awhile

k20

k20

3/4 ton status
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Sep 9, 2001
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Mineral Springs, NC
Ok, so its not my problem, friend of mine got an 82 motorhome, based on the g30 van. 350, th400, blah blah blah. Anyway, it had been sitting along time, got it home and it was pretty much out of fluid, brake system looked horrible, etc. Anyway, we replaced all the soft lines, front calipers, pads, rear wheel cylinders, shoes, master cylinder. Did a carb/manifold swap, dual exhaust, then we took it for a drive. Ran great stopped great.

After hes driven it awhile it has gotten to where the brakes will start dragging, then it will pretty much slow to a crawling stop with the brakes smoking. All 4 are locking down. Once you crack open a bleed on the front, it will shoot out a stream of fluid, do one of the rears, and you are ready to rock for a little while, then it will do it again.

We have bled the system a ton of times, replaced the Dot 3 fluid with dot 4 (hoping the fluid was getting too hot, maybe from the new exhaust being close to the brake lines), also replaced the proportioning valve.

Thought we had it fixed, then today it did it again.

I cut the power steering belt off the bastard to see if it did any different (since its hydroboost) and it made no difference.

He warrantied out the new master cylinder thinking maybe that was the problem, no dice, still does it.

We dont really want to buy a new hydroboost if thats not the problem, but thats pretty much the only thing in the system that hasnt been changed and we are at the end of logical thinking with this thing. Does anyone else have any ideas.....please?
 
Sounds like wrong master cyl or wrong pushrod from booster to master. Get the brakes to lock up and then unbolt the master from the booster just a little see if it frees up. If it frees up you can change the pushrod length.
 
I am the guy having the issue, so I figure I'll jump in and comment.

One thing k20 left out is that I put header wrap around the exhaust where it came into close proximity with the brake lines (front passenger hard line and the rear hard line), and I put thermal foil tape around the brake lines in this area. I flushed the entire system of DOT 3 and switched to DOT 4. The old DOT 3 appeared too dark and slightly cloudy, so I thought perhaps this was caused by heat. To add to the header wrap, I added some thin metal heat shields in this area as well. Still had the problem almost unchanged from before.


I thought about the pushrod being a potential problem, but why would it be wrong? And why would it slowly extend itself to the point of locking down the brakes while driving? It seems like if it was wrong and always extended too far, I would have the brake problem all the time and not just after driving for a while. Perhaps the calipers/drums are very slightly engaged all the time, causing heat to build up, which in turn is building pressure at the brakes themselves, which is then locking them down?

I will try to lock them down and back the master off of the hydroboost to see if this lets off the pressure.
 
You replaced the master cylinder, it is wrong. As the brakes heat up the fluid expands and that fluid has to go somewhere. It is supposed to vent pressure back into the fluid reservoir via a vent hole in the m/c. If that hole is blocked than the only other places for the fluid to expand is the brake caliper pistons and rear wheel cylinders. Something is pushing in the piston on the m/c just enough to block that vent hole. Warranteeing one wrong m/c with another wrong m/c is not going to fix the problem. You need the correct m/c.

If the parts store still has the original m/c compare it to the new one. I'd bet the old one is deeper at the pushrod contact than the new one. Could be as little as 1/16". Instead of looking for a m/c from a G30 van I'd be looking for a m/c from a cab/chassis with hydroboost.
 
All of the above is correct...I'd guess the master cylinder is wrong--there should be a slight amount of free play in the brake pedal ,the push rod that operates the master cylinder cant be tight against it ,if so,once the brake fluid expands the barkes will self apply...
 
well, I looked when we swapped and the masters appear identical. RPO list is for the JB-8 brakes and got a master for the JB-8. Would there be a difference for JB8 between a cab & chassis vs a G30?
 
I drove it about 25 miles today at highway speed. Half way through the test drive I stopped and I could smell burning brakes, but they weren't "locked down" where I couldn't roll freely, so I kept driving. By the time I finished the test drive, they still weren't locked down to the point that I couldnt move but I could tell they were grabbing a little bit.

I went ahead and pulled the master off of the hydroboost but I didn't notice any release of pressure like I do when they lock down and I pop the bleeder. This doesn't really tell us much because like I said they weren't locked. I will try again if I can get them to literally lock down again like they have in the past.

I did pop the bleeder with the master off of the hydroboost and I had some fluid drain freely from the bleeder screw, but it didn't shoot out like it does normally when it's locked up.


I'm about fed up with this, and I am thinking that if I purchase a G30 JB8 hydroboost to go along with my G30 JB8 Master Cylinder, this should solve the problem no matter what it is unless our theory is just totally incorrect and it's something else causing the pressure.
 
If your gunna just throw money away is take the pushrod out of the booster and shorten it like a 16th of an inch and see if you still get burning brakes
 
Better yet, see if you can tell if the pushrod is pushing the cylinder. It should probably be slightly loose when everything is bolted up.
You probably can feel with the pedal. If you push it by hand, there should be a slight amount of movement before the rod hits the cylinder.

Slight off topic here, Ford had a problem one year with the master cylinders on their hydraulic clutches.
If you replaced it, odds were you started burning up clutches because they would not completely release.
They came out with an adjustable rod that screwed into its self with a lock nut.

I'm wondering if there is a part mismatch somewhere here. Maybe they make different push rods for different cylinders.

The other two possibilities are: Bad spring on the brake pedal letting the weight of the pedal rest on the MC, or a problem with the E-brake letting the shoes drag back there and build up heat in the system.
 
I'm guessing that the critical length is too long.
A little short should be harmless as long as you have enough travel.

The clutch rod was under tension at all times, and kept the pressure plate from clamping all the way.
When they got it set right, you could take hold of it under the dash, and there was small amount of play. Before, it was stiff with no play.

I suspect your setting would be the same.
With the brake pedal at rest, you should have some slack in the rod.

If the rod has slack, then it may be the booster. But it sure sounds like the master cylinder is being kept from returning all the way and keeping the return port blocked.

When the brake fluid expands, it has no where to go and the wheel cylinders start moving.
 
You know, I may be wrong about that adjustment.

I had a bunch of folks in the office, got bored, and replied to your post to take my mind off what I was supposed to be doing.

Now that I have a minute, it dawns on me that your system might have two rods. One from the pedal to the booster, and one from the booster to the MC.
If so, then you cannot tell anything by looking under the dash.

You may be able to tell by taking the rod between the two out, putting a small amount of clay or playdo on the end and putting it back together.
Then pull it back apart without pressing the pedal to see if there is any slack.
 
OK here is an update. I got a new hydroboost to install this weekend (Actually K20 is going to install it because I am going out of town ;-) )

Today, I drove it again attempting to lock up the brakes. I could tell they were dragging a bit so when I got to a stopping point I parked and sure enough the brakes were all hot and I could smell they had been burning. I went ahead and took the master off of the booster and popped a bleeder. Fluid came out when I loosened the screw, but it didn't shoot out like normal when they lock down. Is it normal for some fluid to flow freely from the bleeder even if the pedal is not depressed?

When I put the master back on, I put 2 washers on each bolt holding the master onto the booster. This spaced it out a bit and gave me some free play in the pedal which I did not have at all before. The brakes felt more natural this way and I think that I have finally pinpointed the problem.

Now the question will be if the new hydroboost will require the washers to space it out or if there is just something wrong with mine causing it to always be slightly depressed. Hopefully it will work out of the box, but I suppose we can shorten the pushrod somehow if necessary. Otherwise, I might be stuck with the washers for the time being.

We shall see what happens!
 
It is normal for fluid to flow from an open bleeder screw. It's a bleeding method called gravity bleeding.

Washers between the m/c and booster, nice quick fix. :waytogo:
 
I have seen some brake boosters that had a plastic spacer/gasket between the master cyl. and the booster (on vaccum boosters that is-)...not sure if ant hydroboost had them..maybe if one was missing that would account for the lack of free play in the push rod?..anyway,the washers should work OK...my friend at the junkyard converted his 65 Lemans to a dual master cyl and he had the same trouble as you,the brakes locked up after a 5 mile ride!..he just used bolt cutters to shorten the push rod a bit and ground it smooth again!..(he's a lot braver than me!)--but he never had any troubles with it again after that...
 
(he's a lot braver than me!)
Click to expand...

No, he just works at or owns a junkyard. When you know you have plenty of replacements, its not much of a risk.
 
Ok, well, finished up the hydroboost install last night, same damn problem. Well, I say the same problem, I didnt feel like driving it and having em lock down that late at night, but the pedal is still rock hard. After he spaced it out, I drove it once I did some work on the carb, before I replaced the master, and the pedal is still hard, just you have some throw for it to take up the slack between the pushrod and the back of the master.

Im kinda starting to wonder if perhaps its a flow problem. Reason I say this is when you step on the pedal you have to put a fair bit of muscle in it to get it to stop, the pedal is hard, etc. Almost wondering if b/c of the underhood heat and a lack of flow, if its not overheating the fluid in the booster (which we know gets wayyy hotter than the one in K20 or my dually), and then as that fluid expands it applies the brakes.

Another reason for this train of thought, is that on k20 or my dually if I disconnect the lines to the booster from the PS pump, I get some fluid flow out of the lines & out of the booster. Yesterday when I disconnected the lines on his, it was all pretty much dry.

Also his powersteering seems to be wayyyyy oversensitive. To the point where you barely turn the wheel and its a lane change and if you breathe on the steering wheel you can pull a U-turn.

Long post I know, but does the thought of poor fluid flow or something wrong with the PS pump make sense to anyone else?
 
Excessive p/s pressure should not effect the mechanical steering linkage ratio from steering wheel input to steering box output. Excessive pressure in the booster would only apply the brakes harder.

Fluid heats up, it expands. In the p/s system it is constantly flowing. As it heats and expands it flows into the p/s reservoir (res). This is the job of the res, to hold the extra fluid as it expands. In the brakes system, it does not flow. The fluid is merely a mechanical linkage between the brake pedal and the caliper piston or wheel cylinder. As the fluid heats and expands it bleeds off into the brake fluid res on top of the m/c. The vent hole is the furthest back towards the firewall. It's been nearly 20 years since I was in school for this stuff so bear with me, I may not remember every detail. The slightest pressure on the m/c can push the piston (inside the m/c) past this vent hole, blocking it, preventing fluid bleed off into the res.

At this point the brakes have been heated so bad the pads/rotors have been glazed over. Stopping power is dramatically decreased. You may be able to save the rotors by having them turned on a lathe, but the pads will need to be replaced.

Unless, of course, the m/c is still wrong. If the piston diameter inside the m/c is too small it will not apply enough mechanical advantage to the caliper pistons. Or is it the other way around, smaller piston=more mech adv? Again, been a long time since school. Has anyone compared the original m/c to the new one(s)?
 
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