has anyone successfully fixed a rear wheel lock-up scenario under hard braking? (1 Viewer)

[roma042987]

I suppose this question ONLY applies to 40 owners with disc/drum set ups. I've searched, and have experimented personally with Wilwood, factory used, and other brands of prop valves for the rear brake circuits with little success. The aftermarket adjustable prop valves help, but don't entirely correct the problem. I want to go ahead and state that this IS NOT a warped drum, weak caliper, bad master cylinder etc etc. problem. This is a design fault in my opinion. Later in production Toyota installed Load Sensing Prop Valves on most Land Cruisers in my opinion to correct this systemic issue.
My question is has anyone solved this issue with a combination of prop valves or what not that I haven't tried yet?

Just for back story this is a 77 fj40 with a late model (single wheel cylinder per wheel) rear axle. All the brake components including the calipers, rotors, drums, wheel cylinders, master etc. are all relatively new. I guess I should go on to say that I've put together several fj40 with the same components and they all have ultimately braked really well with the exception of this attribute. I guess I'm just at a point where I would like to put some bandwidth into a correction for it instead of adapting my driving habits like I have done over the years. My searches haven't yielded anything positive with the most recent one yielding someone giving up on solving the problem.

 

[B y r o n]

What happens when you back off the adjustment for the rear shoes?

 

[roma042987]

What happens when you back off the adjustment for the rear shoes?

They're adjusted properly. With the single wheel cylinder set up there is no guess work. Loosening them may yield a "band-aid" improvement while sacrificing parking brake performance. Not my goal here.

 

[4x4veteran]

Is it one corner locking or both? Both could mean the flexible line could be bad and not letting the pressure on the cylinders release.

 

[roma042987]

Is it one corner locking or both? Both could mean the flexible line could be bad and not letting the pressure on the cylinders release.

It's both. Lines are relatively new. The last three 40s I've put together have been this same way. Same parts. It's not a component fault.

 

[peesalot]

Its a 90 inch wb fairly heavy vehicle, my 72 bronco did it to. Fine tune the adjustments and get it as close as you can.

 

[mattressking]

Yep, that's all it is, super light and short wheelbase. Mine locks up under hard braking with 35s whereas my 60 with essentially the same exact combo wouldn't lock up. You'd have to add more weight to the truck or add-in ABS

 

[raisinhell2]

Port valve is stuck

 

[B y r o n]

As @mattressking mentioned, have you tried adding say 500 lbs. in the back and see if it still locks the rear brakes?

 

[Skreddy]

Are you using the 77 master cylinder or later model? If using the 77 but then the single rear wheel cylinders, sounds like you effectively halved the fluid volume needed for the rear to work but are still providing the same amount?

Is the proper port on master cylinder going to the proper axle?

I could lock up my rears before the fronts, but then I backed off the rear adjusters until they locked up all 4 evenly (77 style double rear drums and 4Runner front discs). I don’t consider that a band aid fix, I consider it proper adjustment for the system.

 

[brian]

So you have mixed oem parts, like the 77 front components with later year rear components....and you think this is a design fault of toyota that they lock the rears first?

 

[cruisermatt]

So you have mixed oem parts, like the 77 front components with later year rear components....and you think this is a design fault of toyota that they lock the rears first?

FJ60’s have the same front brakes. I think @mattressking is on it

@roma042987 what master cylinders are you using? I like to use the FJ60 / 80-84 FJ40 master cylinder

I did a 40 with this setup but with stretched wheelbase and it wasn’t an issue.. about to do two more but with stock wheelbase. Should be interesting to see if it’s different

 

[pb4ugo]

Keep in mind, when you back off the rear brake adjustment it effects pedal height.

 

[roma042987]

Its a 90 inch wb fairly heavy vehicle, my 72 bronco did it to. Fine tune the adjustments and get it as close as you can.

Yeah I get it I get it. That's what I've always done in the past. I'm just wondering if there's some magic combo or prop valve I'm not privileged to know about. I didn't intend to start another endless thread with a bunch spit ball ideas of what "might" be wrong hence why the title is pretty specific.

 

[roma042987]

Yep, that's all it is, super light and short wheelbase. Mine locks up under hard braking with 35s whereas my 60 with essentially the same exact combo wouldn't lock up. You'd have to add more weight to the truck or add-in ABS

Agree Nate. I'm just looking for a magic trick. I'm seriously considering installing the reproduction o.e. valve followed by the Wilwood unit if the o.e. valve alone doesn't remedy it.

I'm curious if anyone on here has installed Roger's valve and what the results were.

 

[roma042987]

Port valve is stuck

Did you mean prop valve? I don't think three new prop valves in row were all stuck. Adjustment yields changes in bias just not enough.

 

[roma042987]

As @mattressking mentioned, have you tried adding say 500 lbs. in the back and see if it still locks the rear brakes?

No, ....im sure it would help or solve the issue but I don't plan to ride around all the time with it loaded down.

 

[roma042987]

Are you using the 77 master cylinder or later model? If using the 77 but then the single rear wheel cylinders, sounds like you effectively halved the fluid volume needed for the rear to work but are still providing the same amount?

Is the proper port on master cylinder going to the proper axle?

I could lock up my rears before the fronts, but then I backed off the rear adjusters until they locked up all 4 evenly (77 style double rear drums and 4Runner front discs). I don’t consider that a band aid fix, I consider it proper adjustment for the system.

80 series disc/drum non abs master.

The ports are a good question....I've been reading some threads with mixed opinions as to which port is for which circuit. I ran my front port to my front circuit.

Adjustment on the rear isn't the issue. I promise

 

[roma042987]

So you have mixed oem parts, like the 77 front components with later year rear components....and you think this is a design fault of toyota that they lock the rears first?

Yes. My 40 was like this when it was stock as well. As a matter of fact I think every 40 I've owned that had working brakes locked the rear up first.

 

[JohnnyC]

Could try the larger front calipers from a 4Runner … give you more stopping power up front

Had you got a good grippy semi metallic pads on the front? Your not running ceramic up front are you

And the rear shoes use the worst lol

I don’t think you can get ceramic for the shoes but cheapo hard organic shoes would have less bite if your running semi in the rear