Brakes Dragging When Hot

[theastronaut]

Ever since I got my '66 C10 back on the road I've noticed that the brakes drag once they get some heat in them.

Everything except the hard lines are new. The hard lines were in good condition, no rust. I filled them with brake cleaner and blew them out with compressed air multiple times until the cleaner came out clear. The lines are free flowing with no restrictions.

It has a CPP front disc kit, stock rear drums, and a stock replacement single circuit master cylinder, no booster.

I have verified that there is plenty of pushrod free play multiple times. I even added 1/8" shims between the M/C and firewall and drove around and the brakes still held pressure once they warmed up. I can unbolt the M/C completely and it still holds pressure until it's cooled off.

At this point I'm thinking the master cylinder is machined wrong internally, like the piston can't fully return? Has anyone else experienced this? I've searched and found cases where either the booster pushrod or the pedal pushrod was too long, but I've ruled that out by unbolting the master cylinder completely so that it's not contacting the pushrod at all. If it were the caliper slides binding I'd expect one side to be worse than the other, this seems like it's holding pressure in the lines- both wheels are equally hard to spin if I jack the truck up after driving it, and I would think the slides would bind when cold or hot?

The problem has limited me to only driving the truck short distances so I'd like to get the issue solved soon.

[The Rocknrod]

How old are the brake hoses?

[theastronaut]

Everything is new except the hard lines.

[jayoldschool]

You've already added the front discs. Swap on a dual master.

[EricS76]

You need a disk/drum dual master. The single circuit master is made to work with drum/drum brakes.

[theastronaut]

The last ‘66 I built had the exact same brake setup and the brakes didn’t drag with a single circuit master cylinder. I’m planning on swapping to a dual m/c but with the same bore size.

I had the same problem happen on my ‘64 VW about ten years ago, but it started after I changed a flex hose and bled the brakes. On the VW I could hear the piston in the master cylinder unstick and pop back and release pressure.

[SkinnyG]

When this has happened to me with all new brakes, it's because I had the master cylinder pin adjusted too far, and when things got hot and expanded, the fluid return port in the bottom of the reserviour got sealed off and heat just pressure-applied the brakes.

[theastronaut]

Originally Posted by SkinnyG

When this has happened to me with all new brakes, it's because I had the master cylinder pin adjusted too far, and when things got hot and expanded, the fluid return port in the bottom of the reserviour got sealed off and heat just pressure-applied the brakes.

That’s what seems to be happening here but I can unbolt the master cylinder so the pushrod is completely disconnected and it still holds pressure.

[bigmoe]

Was it a new master? Could be sticking.

[bhap]

Open the bleeder on the caliper when they are sticking and see if the brakes relax. If they do, your issue is "up stream", if not, I'd suspect the caliper pistons. A caliper rebuild kit is cheap and easy to install. Make sure both pistons and bores are clean and free of rust and pitting. Good luck!

[pdxhall]

Depending on how you cleaned the hard lines after you ran the brake cleaner and blew out with air, could you have contaminated the brake fluid?

[SkinnyG]

Originally Posted by theastronaut

That’s what seems to be happening here but I can unbolt the master cylinder so the pushrod is completely disconnected and it still holds pressure.

Could be a bum master with the wrong pistons/seals/springs in it.

[theastronaut]

Originally Posted by pdxhall

Depending on how you cleaned the hard lines after you ran the brake cleaner and blew out with air, could you have contaminated the brake fluid?

I used compressed air, we have an air drier setup and filters so the air is clean. Same air I paint with.

Originally Posted by SkinnyG

Could be a bum master with the wrong pistons/seals/springs in it.

That's what I'm leaning towards. I'll swap it out and report back.

[pdxhall]

I wasn't talking about the air but the cleaner may not have removed all of the contamination in the line.

[62c30]

I had heard drum brake master cylinders hold a little pressure to keep the shoes closer to the drum. I had a similar thing happen on a 66 mustang I put disc brakes on, a new disc brake master fixed it.

[Braunschweiger]

I also know it from the Mustang, I found this here:
http://67-72chevytrucks.com/vboard/s...d.php?t=734100

and this in a German tech book:
story
This is what a good brake system looks like in 1960. There are neither disc brakes nor brake boosters. No trace of a two-circuit system either. In the last two decades, vehicles in the lower price segment have overcome cable brakes with all their adjustment and service life problems.


Pascal's law
Pressure is evenly distributed in connected systems.

The hydraulics guarantee absolutely equal power distribution. At that time, one did not think about the possible failure of a brake circuit. The duplex drum brake at the front keeps the actuating forces within limits. A bottom valve on the master brake cylinder ensures a certain pre-pressure of approx. 0.3 bar in the system, so that the brake shoes are already slightly in contact and thus shortens the brake's response distance.
The USA was further ahead, but it fits here.

Your first rebuild may not have had that residual pressure valve, so it may have worked

[theastronaut]

Originally Posted by 62c30

I had heard drum brake master cylinders hold a little pressure to keep the shoes closer to the drum. I had a similar thing happen on a 66 mustang I put disc brakes on, a new disc brake master fixed it.

Looking at the diagram in the factory service manual, there is a valve in the stock master cylinder. I always thought that a residual valve was only needed in brake systems where the master cylinder was mounted low on the frame.

Seems like I need either a disc/drum master cylinder that has a valve in the rear circuit only, or a master cylinder with no valves and an external residual valve for the rear drums.



https://help.summitracing.com/app/an...idual-valve%3F

[theastronaut]

Originally Posted by Braunschweiger

I also know it from the Mustang, I found this here:
http://67-72chevytrucks.com/vboard/s...d.php?t=734100

and this in a German tech book:
story
This is what a good brake system looks like in 1960. There are neither disc brakes nor brake boosters. No trace of a two-circuit system either. In the last two decades, vehicles in the lower price segment have overcome cable brakes with all their adjustment and service life problems.


Pascal's law
Pressure is evenly distributed in connected systems.

The hydraulics guarantee absolutely equal power distribution. At that time, one did not think about the possible failure of a brake circuit. The duplex drum brake at the front keeps the actuating forces within limits. A bottom valve on the master brake cylinder ensures a certain pre-pressure of approx. 0.3 bar in the system, so that the brake shoes are already slightly in contact and thus shortens the brake's response distance.
The USA was further ahead, but it fits here.

Your first rebuild may not have had that residual pressure valve, so it may have worked

.3 bar is right at 4 psi, and disc brakes don't need residual valves unless the MC is mounted below the calipers, and even then only 2 psi is recommended so that's probably why they're dragging. I'll try pumping them a few times to build pressure without driving it to see if its the valve holding pressure or if it's heat related. It does seems to get worse the hotter they get.

[theastronaut]

I just jacked the truck up, verified that the wheels spin when cold, pumped the brakes, and the brakes are dragging with no heat. So it's definitely the internal residual valve. I'll order a new disc/drum master cylinder which should fix it.

[jayoldschool]

Originally Posted by theastronaut

I just jacked the truck up, verified that the wheels spin when cold, pumped the brakes, and the brakes are dragging with no heat. So it's definitely the internal residual valve. I'll order a new disc/drum master cylinder which should fix it.

Good! Glad you're getting it figured out. Don't forget to make sure that the front brakes are connected to the correct port on the new master.

[theastronaut]

I remembered that I had a new combination valve kicking around so I looked up what's inside them and decided to use it with some modifications.

The front circuit has a hold off valve to delay applying pressure to the discs until the rear has enough pressure to overcome the shoe return springs. Without this the discs would start working immediately and the drums would only work under harder braking. The hold off valves helps even out f/r braking effort.

The center is a shuttle valve that moves with excess fluid flow, like if one circuit fails, which activates a warning light switch. It also blocks off the failed circuit to prevent fluid loss. I deleted this since I also want to delete the rear proportioning valve.

The rear prop valve is fixed so who knows if it would work correctly in the truck. I'd rather have an adjustable valve to compensate for different tires, different brake pad compounds, or brake shoe replacements that might have a lower or higher coefficient of friction. I decided to delete it but it's a weird 3/4-20 thread pitch so I had to machine a plug, I couldn't find anything readily available in that thread pitch.

The new 1" dual circuit master cylinder, Wilwood adjustable prop valve, and 10 psi residual valve for the rear drums will be here later this week.



The hold off valve.




Shuttle and switch. I'll plug the hole the switch was in.




Prop valve.




Machining the prop valve delete plug. I tapped it with 1/8" pipe threads to accept a 3/8-24 inverted flare fitting... my lathe skills aren't up to machining an inverted flare straight into the plug