Front Brakes still pulling after disc brake swap (4 Viewers)

[72Roper]

Have a 9/72 built fj40 that had front drum brakes when we purchased it.

Pads on front driver drum brake consistently wanted to drag even after changing the wheel cylinders, pads on driver and passenger sides. Multiple attempts to adjust (based on instructions on this site) as well as bleeding and re bleeding many times)

Front driver side brakes still kept dragging, would then get hot and basically lock the brake on that side up to the point of not being able to drive forward.

We pulled front drums and replaced with a disc conversion. After the conversion, the front driver side caliper now wants to drag and act the same way the drum did. Drags, gets hot and basically locks down.

We have not yet replaced the master cylinder with a later model disc/drum master cylinder (which we plan to do next) but wanted to see if I could get some thought and input from the group here. I could see the old master cylinder creating issues between front and back if the prop valve is set up for drums in front and back but it seems very odd that it could create issues between driver and passenger sides.

Would love any input or past experiences.

Thanks!

 

[pb4ugo]

There's a couple possible things that could create the symptoms you describe. 1st and most likely is the brake hose at the drivers side knuckle is bad. They can crack or leak internally. The hose can create an internal bubble when the brakes are applied and wont release the pressure at the caliper. I would replace the rubber line on the passenger side too while your at it. Also but isn't likely your problem. When you go to disc brakes using a drum brake master you need to remove the internal residual valve in the master for the new frt disc brake circuit. Drum brakes have the residual valves to maintain some pressure in the lines.

 

[72Roper]

There's a couple possible things that could create the symptoms you describe. 1st and most likely is the brake hose at the drivers side knuckle is bad. They can crack or leak internally. The hose can create an internal bubble when the brakes are applied and wont release the pressure at the caliper. I would replace the rubber line on the passenger side too while your at it. Also but isn't likely your problem. When you go to disc brakes using a drum brake master you need to remove the internal residual valve in the master for the new frt disc brake circuit. Drum brakes have the residual valves to maintain some pressure in the lines.

Thank ls for the feedback. New lines were actually installed for the front drums and then again with the discs. All new hoses in all 4 corners. I’ll
Definitely look into that residual valve in the existing MC. I would imagine that it could possibly have been bad even when the drums were still on and could be causing the same symptoms regardless of drums and/or discs.

Thank you

 

[pb4ugo]

I didn't mention this b4 because you didn't mention anything about replacing the master, but who know what the po did. There's an adjustment rod between the master and brake booster. If it's out of adjustment it can put slight pressure on the masters piston. Over time driving it, it will build pressure in the brakelines if it's not adjusted properly. In the link below the OP was having a similar issue. I think around page 24 he found the problem. The whole thread takes a deep dive in sticking brakes.

Thread 'I can't drive 55'

Oct 3, 2024

I moved this to a separate thread to apply some focus.
My original question is, why is my top speed in 4th gear ~45-48 mph?

Background:
5/75 chassis with an estimated 68-70 F (PO said .5F but others have doubted that).
4 Speed transmission
32" tires

Vacuum advance on the distributor appears to work fine. At idle if I pull additional vacuum, it advances the timing noticeably. I pulled the plugs and they look amazing.

It has a mini-truck conversion to disk brakes in front. When raised the wheels turn easily. I recently rebuilt the knuckles and installed new calipers. (It drove this...

• MatthewMcD
• mph speed
• Replies: 528
• Forum: 40- / 55-Series Tech

• 

 

[72Roper]

I didn't mention this b4 because you didn't mention anything about replacing the master, but who know what the po did. There's an adjustment rod between the master and brake booster. If it's out of adjustment it can put slight pressure on the masters piston. Over time driving it, it will build pressure in the brakelines if it's not adjusted properly. In the link below the OP was having a similar issue. I think around page 24 he found the problem. The whole thread takes a deep dive in sticking brakes.

Thread 'I can't drive 55'

Oct 3, 2024

I moved this to a separate thread to apply some focus.
My original question is, why is my top speed in 4th gear ~45-48 mph?

Background:
5/75 chassis with an estimated 68-70 F (PO said .5F but others have doubted that).
4 Speed transmission
32" tires

Vacuum advance on the distributor appears to work fine. At idle if I pull additional vacuum, it advances the timing noticeably. I pulled the plugs and they look amazing.

It has a mini-truck conversion to disk brakes in front. When raised the wheels turn easily. I recently rebuilt the knuckles and installed new calipers. (It drove this...

• MatthewMcD
• mph speed
• Replies: 528
• Forum: 40- / 55-Series Tech

• 

Thank you. I’ll take a dive into that thread. Where is the adjuster that you mentioned between the MC and brake booster? The vehicle isn’t here with me so I do t have the advantage of walking out to the garage to search for it unfortunately.

 

[pb4ugo]

It's between the master and the booster. There's a measuring tool which can be purchased a most a/p stores. On page 24 post #480 he shows the adjustment rod in the booster with the part of the adjustment tool in the pic showing how much it's out of adjustment it is. The tool measures the depth of the masters' piston in relation to the rod that's in the booster.

 

[royal cake]

Early in my build I forgot the brake pedal adjustment per the service manual and it was locking up the brakes. I second the suggestion to check that.