1974 FJ40 disc brake drag with front FJ60 conversion

[cstav86]

I have an 11/1974 FJ40 with an FJ60 disc brake conversion up front and I’m experiencing some intermittent brake drag on the front discs. The booster is a city racer for a 40 with a 40 series disc/drum master. Master is an O’reilly I believe. I didn’t do the conversion so I don’t know it as well. It’s also got a proportioning valve on it. The last time I was experiencing drag I opened the bleeder and the caliper released and you could spin the wheel freely. Drag occurs on both driver and pass sides so I’m thinking it’s something master related. No drag on the rear drums. Any ideas what could be causing the calipers to not be releasing? This is a sporadic deal and happens only sometimes so I’m looking for some outside help.

 

[Green Bean]

Folks more knowledgeable than me will chime in but isn't that a residual valve and not a proportioning valve? Is it on the front circuit or rear?

 

[peesalot]

There may be a 2lb residual valve in the master cyl. , under the large fitting the brake line screws into., remove if it's there. FWIW I have had calipers get sticky, the piston gets gummed up and they drag, however letting off the bleeder does not solve that so I am leaning toward residual valve.

 

[cstav86]

Folks more knowledgeable than me will chime in but isn't that a residual valve and not a proportioning valve? Is it on the front circuit or rear?

The proportioning valve is for the rear.

There may be a 2lb residual valve in the master cyl. , under the large fitting the brake line screws into., remove if it's there. FWIW I have had calipers get sticky, the piston gets gummed up and they drag, however letting off the bleeder does not solve that so I am leaning toward residual valve.

@peesalot Any idea what the residual valve looks like? Will I need to fish it out with needle nose?

 

[B y r o n]

Disc brakes are supposed to have a 2 lb. residual valve, drum is 10 lb. It maybe that the master is a drum/drum. A 10 lb. residual valve will cause the front brakes to drag.

 

[wngrog]

If his drag is in the front it has nothing to do with the residual valve. That’s in the rear

I’m going to bet the rod between the booster and master is too tight.

Look at this link

https://cdn.shopify.com/s/files/1/1392/1475/files/Brake_Booster_Installation_Supplement.pdf?412

 

[Skreddy]

The booster to master rod has gotten many a 40 owner.

 

[wngrog]

The booster to master rod has gotten many a 40 owner.

I bought that cool SST tool

 

[cstav86]

I purchased a tool to check the clearance. Appreciate the help guys.

 

[Dynosoar]

That looks like a Drum / Drum Master.
Try a FJ60 master

 

[peesalot]

Wngrog, are you saying that a drum/drum MC does not have a residual valve in the front circuit ? Please elaborate.

 

[SteveH]

Residual valves (by design) are for drums, not disks, so they will not be in the front brake circuit. The manufacturer puts them in drum circuits to reduce the latency of the brakes and make them respond more quickly to pedal input (someone correct me if this is wrong).

The booster to master rod has gotten many a 40 owner.

Agreed. I would start there.

 

[cstav86]

Taking y’all’s advice and eliminating one thing at a time. I checked for the residual valve and there is none in the front. No brake fluid came out when I opened it up but there was fluid down in there. Re-bled the brakes and initial shakedown run it’s performing fine. Keep in mind the problem is sporadic though. When my tool comes in next week I’ll verify the clearance on the booster rod.

 

[Dynosoar]

The thing the arrow is pointing to and circled is the residual valve.
Dyno

 

[peesalot]

Nope, it's a little disc. under that piece.

 

[Skreddy]

You can check the booster rod if you have normal calipers, just math. But the tool makes it much easier.

 

[lcwizard]

go with Nolan's advice. Shorten the rod a turn or two at a time. There's a relief port in the master. When then pedal is released,the plunger comes back,
exposing the relief port, allowing fluid to return from the calipers to the master. In the 80s I got a master that, somewhere in production, ports weren't
drilled through. I could drive about a half mile, using the brakes when needed but the brakes slowly began to drag until full lockup.
When I removed the cups you could see where a drill operation had started but did not go completely through. I drilled the ports through and everything worked perfect.
If you're dragging but not eventually getting full lockup you probably have a partial flow back

 

[mattressking]

I just installed one of those boosters on a truck, it definitely needs pushrod adjustment out of the box.

 

[Skreddy]

I know this may have been resolved but for others.

If you don’t have calipers and don’t want to buy the tool, stick a little ball of play-doh in the master cylinder cup, hold the master to the booster then check the play-doh. Was it squished all out: shorten the booster rod. Was it still there and thick: lengthen the booster rod. It’s close enough when the play-doh is squished to the sides and just a thin layer remains where the booster rod contacts.

But calipers or the tool are much better and easier.