Brakes on the 40 Gnob & Heron

[TOY350]

All feel free to jump in but I have been meaning to ask Al & Aric specifically.

Problem: On my 40 the rear brakes lock up before the front. The fronts do work but not very well apparently. We can lock the rears up easily now with the rear disc conversion just like we could lock them up when we had drum rear. Dry pavement has the rears locking and the fronts just slowly coming to a stop. Even in gravel the fronts roll half the time before locking.

Description:

Front brakes mini truck disc conversion. Have bled them well. Pads in good shape, almost new. Rotor appears fine.

Rear brakes are a disc conversion using the Monte Carlo calipers on custom rotors. Work great and have been bled well.

Master cylinder is a rebuilt non-US FJ45 drum/drum 1-1/8 bore dual resovoir. Rebuilt by me with an OEM kit after we thought the original master was the problem. It wasn't.

Booster is a Toyota Supra from Napa that was replaced during this process because the brakes were sticking with pressure and sometimes almost locking after repeated use. The new booster cured this problem.

Proportioning valve is one from Summit that is on the rear line coming from the master cylinder. Adjustable from 0% to 70% reduction. Has no noticeable effect on allowing the front brakes to lock up first.

Brakelines all appear fine and I can bleed all corners without a problem. All the fluid has been cycled out and is clean.

At this point we are guessing at needing to rebuild the front calipers (we have OEM kits), turning rotors and replacing pads although they appear fine and maybe replacing the soft lines from axle to calipers. We went by BJ's Brake Shop in GP and he didn't have a solution.

 

[2ndGenToyotaFan]

I've got to vote for the front calipers too.... Thanks for asking me! Hehe Everything else sounds new/excusable.....

 

[Ebag333]

If you can't lock up the fronts before the rears even with the proportioning valve closed all the way, you have a serious problem.

I would also check to make sure you don't have a kinked line. If a hard line got pinched/kinked, that would mimic sticking calipers. You could use a pressure gauge to test.

Not sure about the master cylinder you got, but I know with some if they are misadusted it won't open up all the way. If the line for the rears opens up before the front, the front line might not open all the way.

 

[TOY350]

Good points guys, the more things I can check the better chance of finding the problem! And no offense to any of you out there; I was just targeting Gnob's extensive mechanic background and Heron's impressive geniusness.

 

[gnob]

my first thought would be with the master cylinder.
here is why.
a disc and drum require different residual valves.
if i were to throw a part at it i would try a master that was meant to work with those disc brakes.

here is my thinking, the amount of fluid needed to move the drum wheel cylinder is significantly less that of a 4 piston caliper.
and with an incorrect residual setting, the pistons are being allowed to return too much.

the other two things you covered, soft lines and bad caliper seals or partly seized calipers.

my black 4runner has a similar symptom, which is kinda s*** pedal feel and rear lockup (although i removed he stock LSPV and am running without one) but on cold days the pedal feel is better, leading me to think that the soft lines are harder when its cold out allowing for less expansion and better pedal


just so you dont think i am http://www.allprooffroad.com/8489brakeupgrades/263

 

[TOY350]

Please explain the residual valve. I know on the drum setup there is a small "hat" with a tiny hole in it and a spring which keeps a low amount of pressure on the line which keeps the shoes close to the drums. We pulled both of these out of the master since on a disc setup they will cause excessive drag and wear. The residual valve must be something different?

 

[gnob]

Pirate4x4.Com Bulletin Board

here is some reading.

like i said, i could be mistaken about the valve.

i would think that the master is designed with the rest of the system in mind.

 

[Heron]

Not having any experience with the model braking components involved, I would agree with gnob's assessment.
For what it's worth some input.
1. Are the rear brakes locking up under heavy peddle pressure or maybe grabbing at low peddle pressure.
2. If the truck has been sitting with old brake fluid, the caliper pistons have a tendency to size.
3. New master cylinder displacement compatibility. (rear m/c piston to large for rear brakes?)

 

[quatrocantos]

Tell me what is best, the stock drum or the disc for the rear brakes? i was just decide if I should. By the looks of it why change if it will just cause trouble.

 

[Ebag333]

Tell me what is best, the stock drum or the disc for the rear brakes? i was just decide if I should. By the looks of it why change if it will just cause trouble.

Disk provides a huge amount of additional stopping power. As long as they do not overpower the front, you want as much stopping power as you can get.

Normally you can easily keep the rears from overpowering the front by putting in a proportioning valve.

 

[TOY350]

The simplicity of the disc rear is worth the upgrade. No longer trying to adjust the wheel cylinders or dealing with them leaking. No drying out the shoes after going through water. My problem is with the front end since it acted the same in the rear with the drums as it does now with the discs.

 

[TOY350]

The link to Pirate didn't work for me. Wouldn't mind reading up further.

 

[TOY350]

Not having any experience with the model braking components involved, I would agree with gnob's assessment.
For what it's worth some input.
1. Are the rear brakes locking up under heavy peddle pressure or maybe grabbing at low peddle pressure.
2. If the truck has been sitting with old brake fluid, the caliper pistons have a tendency to size.
3. New master cylinder displacement compatibility. (rear m/c piston to large for rear brakes?)

1. Rear will lock with moderate pressure. We can slow down with light pressure or lock them with more force but we don't have to force the pedal down to lock them.

2. I think we are going to pull and rebuild them to see.

3. 1 1/8 bore displacing same amount of fluid front and rear is my guess. Originally on a FJ45 drum/drum HD 70/80's Cruiser.

I think we need to rebuild reassess.

 

[Heron]

Just so there is no misunderstanding of my earlier comments.
I feel the possibility of a front caliper problem is more likely than a problem with the rears.
A few years ago I had a similar problem with no front brakes. The rig had been siting outside all winter with no use. the pedal seemed firm, the rears would lock up, but no joy on the fronts.
Please note I am not recommending this but this was my hackers approach to the problem.
Drove out to the black top and started out at 20 MPH and jammed the brakes on. No change. tried it again at 30 MPH and really jumped on the pedal. Bang I had front brakes again, drove to BJ's and replaced both calipers with new units.
May have been able to accomplish the same thing without the black top exercise but was difficult to evaluate on the gravel.
Al

 

[TOY350]

I hear you Al. We already have the rebuild kits for the front so I think pulling them and rebuilding them no matter what is a good choice since we don't know the history of the calipers. Probably should pull rotors and check them or just turn them anyway. Repack some wheel bearings while in that far and replace the rubber lines with new ones. After that if we still have the problem (also check the hard lines for kinks or crushes) then the last piece will be an 80 master.

 

[gnob]

sorry about the link, you cant use search if you arent a member and all that jazz.
anyway the gist was that a well designed system wouldnt need a residual valve.
residual valve more used in a place where the master is below the caliper/cylinders.

can you "pump" the brakes up? or is the travel and feel right, it just no stop?

i have seen loose wheel bearings allow the disc to push the pistons into the caliper further than norm. this causes excessive travel though.

 

[TOY350]

The brakes feel good although my dad says occasionally they will hold firm while stopping then the pedal will travel just a bit further.

 

[Kief]

Residual valve on the drum brakes cylinder is almost like a check valve and keeps the drum pad closer to the drum resisting the springs that pull them away from the drum. Valve can be disabled. Cruiser drum brakes have two cylinders at each wheel and both shoes "cam" into the drum vs. just one on other rigs.Better design than most brakes of the era but the adjusting sucks and is hard to keep straight on opposite side with right hand and left hand. Disc Brakes don't have the springs that pull them away from the disc and don't need the pressure maintained to keep them from pulling away from the disc.
If the rear calipers have a much smaller volume than the fronts they will closer faster or if the master is pumping a larger volume to the rear and the proportioning valve may not compensate, also if the hydraulic pressure is proportioned right but say the rear calipers apply more pressure to the pads (different mechanical advantage, more/less surface area, floating style vs. 2 or 4 cylinder calipers) then its sort of like applying the same amount of pressure on a simple lever/fulcrum and moving the fulcrum.
I am probably not communication this well but you will have to redesign the system to work right and not rely on the proportioning valve. I have heard of people using 4 runner or 80 series masters that were from all disc rigs..
Remember, I'm the guy that had his 40 swap ends and roll leaving me wondering if it was 'cuz my out of adjustment rear brakes blew out..

 

[TOY350]

Looks like we might have the calipers pulled this weekend to rebuild and assess the possibility of the problem. I understand the input from everyone on different brakes systems and not matching properly and such. But this is a tried and true system: Toyota P/U front disc swap and the same rear disc setup Man-A-Fre sells for the rear swap. Only other thing I see is the master as pointed out but with it supposedly pushing the same front and rear I would expect the fronts to respond much better than they are.

 

[Ebag333]

Only other thing I see is the master as pointed out but with it supposedly pushing the same front and rear I would expect the fronts to respond much better than they are.

Hooking up a pressure gauge will quickly rule out the master cylinder, and show exactly how much is being pushed out.


First result in a Google search:
SSBC Brake Pressure Gauge Kit

I'm sure the local auto shops have 'em. Don't recall seeing one at Harbor Freight....