Weird problem with my front brakes. Can you help?

[Wanderlost_Overland]

I have a weird issue going on with my front brakes. I hope someone else has had a similar problem and can lend a helping hand.
After driving six miles or more, combined highway and stop-and-go, the front brakes progressively get tighter and tighter. After twenty miles or more they almost come to the point of staying locked up. It seems that hydraulic pressure is building up but not releasing, the pedal gets firmer too. The reason I believe this is mostly due to the rear brake lights coming on without touching the pedal. I get home, get out, a few seconds later the brake lights come on, they go off if I tap the pedal but come back on after a minute or two. At that same time when jacked up of the ground I can't spin the front wheels, the brakes are still on. After a long period of cooling down The brakes release, the brake lights stop coming on by themselves, and I can easily push it around the shop.
This is what I have:
This was done by a previous owner, fairly recently by the looks of the parts.
Front disc conversion
Dual master conversion
Stock rear drums, adjusted properly and working perfectly
The pedal linkage is adjusted perfectly
The system is completely bled of air
The brakes work great for the first few miles
Any ideas?

[Bruce]

You don't have any freeplay between the brake pedal push rod and the piston in the MC.
Adjust your pedal stop to give you 1mm at the tip of the pushrod. This equates to about 5 mm at the top of the pedal.
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[KTPhil]

Yes, check the freeplay. Then consider replacing all soft lines; they can "inflate" and maintain pressure even after you let up.

[EVfun]

You mention the brake lights staying on, that suggests to me the brake pushrod may be adjusted a little too long. Related, is that the master cylinder is having difficulty returning all the way when you release the brake pedal. The pushrod length is a quick test. When the car is acting up and you are at home quickly remove the master cylinder pushrod. If your dragging brakes go away you found the problem.

It is also possible that you have internally failing flex hoses, but it doesn’t explain the brake lights.
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[Cusser]

Agree: brake pushrod is adjusted a tad too long. Make it a little shorter.
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1970 VW (owned since 1972) and 1971 VW Convertible (owned since 1976), second owner of each. The '71 now has the 1835 engine, swapped from the '70. Second owner of each. 1988 Mazda B2200 truck, 1998 Frontier, 2014 Yukon, 2004 Frontier King Cab. All manual transmission except for the Yukon. http://www.thesamba.com/vw/forum/album_page.php?pic_id=335294 http://www.thesamba.com/vw/forum/album_page.php?pic_id=335297

[Wanderlost_Overland]

Thanks for the help everyone! The pedal adjustment is correct, maybe even a little too much slack. I'm certain that's not the issue.
For some reason the "piston" inside the master isn't coming back far enough to relieve the pressure. This weekend I'll take it all apart and see what's up.

[viiking]

When releasing the pedal, the hydraulic pressure is released and the fluid is returned to the reservoir. To do this the MC should have a tiny compensating port within it to do this depressurising. If it is blocked you will have this issue. Similarly if the brakes heat up the fluid increases in volume and can jam the brakes on. The heat can be obtained by residual heat from absorbing it from the hot components e.g. rotor, calliper etc. So if the compensating port is partly or fully blocked the brakes do not release.

So after checking all of the other poster’s suggestions e.g. internally swollen brake hoses I would consider this first. It however requires disassembling the MC.

Secondly is the brake fluid correct and not contaminated by accidental mixing up of silicone and non silicone?

Thirdly and less likely is that your calliper pistons are binding/rusty/ damaged. Normally the brake pistons on discs are retracted by the trapezoidal seal cross section which flexes sufficiently to release the pads from the rotors. If the piston is intermittently binding the hydraulic pressure may not be released enough.
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1968 1500 RHD Lotus White Beetle since birth. In the hospital for major surgery
1966 Lancia Flavia Pininfarina Coupe - in the waiting room
Discharged: 1983 Vanagon, 1974 1800 Microbus,1968 Low Light,1968 Type 3

[scrivyscriv]

Brake hoses will cause this 90% of the time. It’s possible the master cylinder has some debris obstructing the compensating port inside but the hoses are more likely. You will almost never see the evidence of failure externally unless it’s a catastrophic failure
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Robert in Memphis
Dünkelgrügen 1967 Java Green bug thread
Engine rebuild thread
If you're ever in the Memphis area, you are welcome to stop by for advice and help.

[Wanderlost_Overland]

So, last night I did a full bleed of the front lines.
Took it out for a long drive today and had no issues at all! I'm assuming the problem corresponds with Viiking's advice about a blockage of the tiny compensating port.
I'm still concerned that it may happen again so, when winter freezes everything over here, I'll rebuild the master and put on new rubber lines.
Thanks for all the help!
BTW, if you're interested in our "build" search us out on Youtube, Wanderlost Overland, and or Instagram.

[viiking]

I'm always in awe of those of you that live in such a cold area that you have to lay up SOME of your cars over winter.

It doesn't happen here. We have the opposite and as I sit here I am awaiting the 44C /111F daily maximum predicted in about 2 hours.

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1968 1500 RHD Lotus White Beetle since birth. In the hospital for major surgery
1966 Lancia Flavia Pininfarina Coupe - in the waiting room
Discharged: 1983 Vanagon, 1974 1800 Microbus,1968 Low Light,1968 Type 3

[wdfifteen]

[EVfun]

VW Beetle dual circuit master cylinders have external residual pressure valves or no residual pressure valves. VW Beetle single circuit master cylinders have a residual pressure valve built inside them.
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