Front Brakes dragging, holding pressure. (1 Viewer)

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Hi All,

I'm chasing a problem with my front brakes. I rebuilt the master 9? months ago and replaced the calipers a few years ago. I changed the fluid when I rebuilt the front axle recently.

The problem: The front brakes drag after pressing the brake pedal. I can release the fluid pressure through one of the bleeders and then I can get the truck to roll by pushing on it. If I press the brake pedal again, it wont roll very easily by hand until I bleed off the pressure again. Has anyone seen this behavior before?

Thanks!
 
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Are you dealing with disc front brakes and did your ride originally have drum brakes? If so the residual valve in the master needs to be removed. Drum brakes use a 5 psi residual valve where disc is usually about 2 psi. If the residual valve is holding more pressure, the brakes won't release.
 
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Its a '78 with original discs in the front and drums in the back. I wonder if I could have swapped the lines on the master?

Edit: Or the residual valves? The lines are correct.
 
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Its a '78 with original discs in the front and drums in the back. I wonder if I could have swapped the lines on the master?

Edit: Or the residual valves? The lines are correct.
20201014_080740.jpg


I have 2 of these although they don't look like valves. They have holes and pegs all over them that seem like they would not seal against the master or the nut that holds them in. The one from under the front line had a tangled up spring which I will fix and re-install.
 

BeerM3

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that is what this tool is used for.

Amazon product
shop around on amazon if you decide to purchase one. The same can be found for $24-28 or even as low at $15 for a even lower-tech version. Search "brake booster adjustment tool"

On the residual valves they're designed to not seal up the port, rather they slowly allow the fluid back into the reservoir(s) at a metered pace as the calipers/pistons retract. It does sound like you might've accidentally swapped em during the rebuild, but I'd also check the booster rod while you're at it. I wish I could tell you which of those is the front disc valve but I'm sure someone on here can.
 
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I ran out of time to bleed them and test. I ended up swapping the valves. I pushed them together to try to figure out which had the most spring pressure and put that one in the rear line. It was hard to tell but I will see if it helps tonight.
 
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I would suggest that to bleed off the residual pressure in the front brakes, for diagnosis, you should crack the line on the master cylinder. If the pressure releases when you do that, you will know that you don't any return restrictions between the master and calipers. An internally failing brake hose can do what you are describing.
 
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Good ideas above, I'm with the group that believes you need to adjust the rod between master cylinder and booster- - -but FWIW, I had the same problem you have, fried my front pads to the rotors at a stop light after a long freeway run, heat required that I had to rebuild the whole knuckles, turned out to be a bad booster that wouldn't release the brake pressure. Booster let the brakes drag just enough that I could still drive the rig, but the drag totally fried the brakes after about a 20 mile freeway trip. Incorrect adjustment at rod between master and booster will do exactly the same.
 
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Thanks for all the ideas! I think I have narrowed it down to the Master. If I crack the line at the master fluid comes running out and I can hear the calipers relaxing and the truck moves easily by hand. I tried removing the "Residual valve" entirely, turning the rod at the pedal as far from the master as it would go (about 1/4"), and finally removed the master from the booster. None of that helped, only releasing pressure at the bleeders or at the master helps. I think that means the master is holding pressure for some reason. I did have to clean up the bore with a Scotchbrite when I rebuilt it so the bore has some slight pitting. The reservoirs are cracking too so I think I'll just order a new one. Do any of you see any problems with this diagnosis?
 
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I had a Honda that did the "no release of pressure on the front discs" . It turned out that the flexible brake hoses had gone bad internally and were acting like one-way valves. Replaced the brake lines and no more problem. Be sure to let us know what the solution was.
 

Goforth

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Had this issue on My ‘67 with 4 drum brakes. Issue was pedal adjustment. I needed about 9” of distance from the firewall to the pedal and was around 5”. Adjusting the pedal back meant I didn’t have hard / stuck brakes
 
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Had this issue on My ‘67 with 4 drum brakes. Issue was pedal adjustment. I needed about 9” of distance from the firewall to the pedal and was around 5”. Adjusting the pedal back meant I didn’t have hard / stuck brakes
I have adjusted the pedal to the limit and it did not help, even removing the master from the booster did not help. I'm assuming that the lines are ok because I can bleed off pressure at the master.
 

pb4ugo

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I have adjusted the pedal to the limit and it did not help, even removing the master from the booster did not help. I'm assuming that the lines are ok because I can bleed off pressure at the master.

I wouldn't make that assumption. An old flexible line with an internal tear can act like a 1 way valve. I'm not saying that's your problem, but could be. The most common cause is the rod coming out of the brake booster.
 
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I wouldn't make that assumption. An old flexible line with an internal tear can act like a 1 way valve. I'm not saying that's your problem, but could be. The most common cause is the rod coming out of the brake booster.
I'm unsure how to diagnose the lines other than what I have done. They are old and crusty like the master so replacement is probably a good idea anyway.
 

pb4ugo

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If the brakes were working properly for a while and this problem arises, then I would 1st suspect a sticking caliper or two. I would open a bleeder and confirm that the caliper pistons will retract with some resistance. Next would be the flexible line running from the frame to the axle, especially if both brakes are sticking on the same axle.
 
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chupacabra
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I have adjusted the pedal to the limit and it did not help, even removing the master from the booster did not help. I'm assuming that the lines are ok because I can bleed off pressure at the master.

Did you adjust the vacuum booster pushrod length while you had the master cyl off?
 

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