has anyone successfully fixed a rear wheel lock-up scenario under hard braking?

[Skreddy]

80 series disc/drum non abs master.
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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

I run the same master and I use the rear port for front brakes. I did a test when bleeding the master and the rear port starts moving fluid first. I could be wrong but I believe the front brakes should start applying first so that’s why I hooked them up that way.

 

[roma042987]

I run the same master and I use the rear port for front brakes. I did a test when bleeding the master and the rear port starts moving fluid first. I could be wrong but I believe the front brakes should start applying first so that’s why I hooked them up that way.

Interesting....I'm leaning toward giving this a quick shot. May be silly to ask but....your rears don't lock up first with it this way?

 

[Skreddy]

No mine all lock up the same time this way. But, like I said, my rears are adjusted a little looser than normal. And by a little looser, I mean probably 3-5 clicks looser than normal? I’m running manual brakes as well (no booster for 1970) but using the 77 rear wheel cylinders. When first installed and rears adjusted how I normally would, the rears would lock first, but just barely, until I backed them off a few clicks.

 

[roma042987]

Some interesting reading if you're here and really want to dive in. Seems like I'm following @dgangle 's well worn path at this point.

Rear Brake lock up

I have done a RDB conversion with Monte Carlo calipers in the rear. The front are stock 40 calipers. All lines and calipers are new. I bought a new master from an 80 series from JT Outfitters. It has the front port come out of the top. The rear has alway locked really easily. I just...

forum.ih8mud.com

Rear disks still locking up during a hard stop

Last year I converted to disks from drum brakes. I am not able to prevent the rear disks locking up during hard braking or emergency stop. I am running '77 FJ40 disks up front and the Monte Carlo in the back, Summit proportioning valve set to less braking (yes, I have checked this a few...

forum.ih8mud.com

Double Pumping the Brakes

I recently did 4-wheel discs on my 71 FJ40. Used JTO - lousy service. Front bracket not ideal. I'll probably rebuild the front in the future using the mini-truck buildout. Am having to double-pump the brakes episodically to keep the pedal from depressing too far. No fluid leaks. Bled the...

forum.ih8mud.com

Brake Caliper reducer for RDB???

Has anyone tried using a brake caliper piston reducer for the GM rear disk brake conversions? (http://www.colemanracing.com/catalog/index.php?cPath=28_1392) Seems to me that if you were to reduce the size of the piston, you would reduce the force exerted, and make it less biased. I have...

forum.ih8mud.com

 

[peesalot]

FWIW I like the OEM dual master, say late 70' variety. Never needed to switch to a newer one. I believe my 40 has the og masters for both clutch and brakes, currently running 4 wheel disc on the oem MC.

 

[h82crash]

I run the same master and I use the rear port for front brakes. I did a test when bleeding the master and the rear port starts moving fluid first. I could be wrong but I believe the front brakes should start applying first so that’s why I hooked them up that way.

I haven't disassembled my 80 cylinder, but if the pistons are separate, then the primary (closest to the booster) builds pressure first. The secondary shouldn't move until the primary pressurizes and starts pushing the secondary piston.

 

[bikersmurf]

To address a couple points that were brought up.
My rear floor is built out of 1/8" steel. There is 90% of a 4'x8' sheet back there. Needless to say, the rear is heavier than stock. I'm feeling lazy now so I'll not convert to imperial, but you'll get the idea... 950kg on the front axle and 1150kg on the rear. Side to side is less than 100lbs difference and is dependent on how full the fuel tank is and passengers. I also am running a SBC which reduced the engine weight by about 200-250lbs over the front axle.

With a '77 - 40 front disc axle, factory proportioning valve, and stock '74 rear drums, the rears didn't lock up easily, but the would if I hit them hard enough... and the fronts wouldn't lock up (except on very slippery surfaces.

'77 - 40 disc axle, Wilwood adjustable proportioning valve, and '77 - 45 (massive rear drums), the rears would still lock up under heavy braking. Skid pad computerized brake analysis results were as good as any vehicle gets per the brake technician.

'77 - 40 disc axle, Wilwood adjustable valve, and '78 Suburban rear discs, the rears will still lock if hammered. The computerized test results still show correct bias front to rear. However, if hit hard enough the rears will come off the ground.

'77 - 40 disc axle, Wilwood valve, '78 Sub rear discs, and 80 series booster & master, the rears will still lock if hit hard enough and if hit fast enough will lock all 4 wheels.

With the current setup, is brake pressure is steadily increased over a short period of time the brakes won't lock. A quick jab (with even just 2 fingers) can lock all 4 wheels on dry pavement.

Conclusions:
Drum brakes will lock much more easily than disc brakes. With a little finesse the rear brakes won't lock up. If the rear comes off the ground (or close to it) from a quick brake application, the rear wheels will lock up. Although extra weight helps, it doesn't solve the issue if the wheels are defying gravity.

4 wheel disc brakes and sensible driving will result in cars running into you because they cannot stop as fast as you'll be able to... so be careful.

 

[brian]

im not buying that they reversed the standard layout of the master cyl.....closest to booster is front brakes, the other is the rear, on abs vs non abs 80 masters.


the only time ive had rear lockup issues with oe front disc/rear drum with all 77 stuff was when the brake(not load) port valve under the master quit working due to be filled with crud

 

[h82crash]

im not buying that they reversed the standard layout of the master cyl.....closest to booster is front brakes, the other is the rear, on abs vs non abs 80 masters.


the only time ive had rear lockup issues with oe front disc/rear drum with all 77 stuff was when the brake(not load) port valve under the master quit working due to be filled with crud

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I was looking for that diagram. Sums it up. Both show 2 piston design and rear piston as #1. #1 means primary to me.

 

[pb4ugo]

To me it means it's the 1st piston that comes out of the cyldr.

 

[JohnnyC]

1990 fj80 brake line routing

 

[4x4veteran]

OK, I misunderstood. When you said the brakes lock up I took it that they wouldn't turn after brakes were applied.
When I first had my FJ40 I had the problem of the rear end wanting to come around.
I lowered the pressure in the back tires and tried to remember to pump the brakes in a panic stop.
The latter is hard to do in said panic.
The old 1970 had drums all around and no proportioning valve.
These rigs are pretty light in the rear and nothing short of disconnecting the rear brakes is going to stop the squalling of tires during an emergency stop.
I now have for wheel disks with a P valve and can still squawk the rears with a hard brake.
Not as bad but still do it.

 

[h82crash]

To me it means it's the 1st piston that comes out of the cyldr.

This is from an automotive engineering site. The image is the same as all the ones that describe ports to brakes. So, if you want your rear brakes to activate before the fronts, hook up the front line to the front port.

Tandem Master Cylinder​

The working of tandem master cylinder.

• When brake pedal is not actuated, the piston remains at their original place, closing the inlet valve of both the compression chambers, which in turn cuts the incoming of brake fluid between both the reservoir or both the reservoir chambers.
• When the brake pedal is actuated, at first the primary piston moves due to which opening of primary inlet valve takes place.
• Initially due to the movement of primary piston compression of the brake fluid inside primary chamber takes place.
• After completion of the compression in primary chamber primary outlet valve opens up and this compressed brake fluid is further sent to brake callipers through brake lines and actuation of the primary circuit brakes take place.
• After the completion of the primary piston movement i.e. at its extreme end, the secondary piston starts moving because of the force applied by the primary piston’s spring which in turn opens the secondary valve and incoming of brake fluid from secondary reservoir to secondary compression chamber takes place.
• This brake fluid is then compressed and after complete compression secondary outlet opens up and this highly compressed fluid is sent to the brake callipers through brake lines and actuation of the secondary circuit brakes take place.

 

[jim land]

I have 16"s so perhaps I may have clearance.

The grinding needed is near the hubs on the 90+ 4runner caliper, 15" or 16" wheels do not matter

For the backing plate you need to cut about 3/8" up and down since the caliper is longer, easy and simple mods

 

[sggoat]

Backing off the shoes increases tolerance between the shoes and drums....more tolerance= more fluid has to move to make contact and friction. More tolerance also yields more pedal travel and more pull required on the parking lever.

Rear brakes are adjusted properly.

A 77 40 has a parking brake that is a drum type brake mechanically connected to the transfer case output shaft--it is not connected to the hydraulic brakes in any fashion

 

[roma042987]

A 77 40 has a parking brake that is a drum type brake mechanically connected to the transfer case output shaft--it is not connected to the hydraulic brakes in any fashion

Refer to post #1. It describes the setup I have.

 

[Living in the Past]

O.E. pads and shoes Johnny. I've considered upgrading calipers but these are reman Toyota calipers and my understanding is that with 4 runner calipers you have to do something to the dust shields...i could be wrong on that.

Do you not want to trim the dust shields? Not staying stock wouldn't think trimming the dust shield would be a big deal.

Yes Eddy all the later trucks including the 40s got LSPV's for this reason.

Not sure that's true. I know all late axles seem to have the bracket for rod to bolt on. But the LWB have the little bracket on weld cover for the diff on FF. That is for hand brake which LWB 40 series never had. I searched Australia late model FJ40 and their is no listing for the LSPV. Not sure any SWB 40 series ever had one. When I check the LSPV it shows 10/82-10/84 BJ45 FJ45 HJ47 ALR, EUR.

When I had an opportunity to get the axles from a wrecked 84 HJ47RV from Australia I ended up pulling all the brake lines, LSPV and the master cylinder. Figure it would be easier adding late axles to my early FJ45 not trying to reinvent the wheel to balance the system Bought the driveshafts and axles before it went to pick and pull section. Luckily only thing the guys who pulled the parts cut was the rod for LSPV. At least I ended up with both ends.

Since I've managed to source some late model 40 series rear axles with a single wheel cylinder have a early 80 series and late eighties mini truck booster and master cylinder. I have a few 76-80 dual diaphragm setups and knowing very little of the difference between the old two wheel cylinder and later self adjusting single wheel cylinder thought mini truck and early 80 series booster/master cylinder may work better. But who knows. Used parts were cheap and nothing else good to experiment with.

 

[raisinhell2]

Nice gaucho I got me one of those as well

On a dual 40 master it is definitely front res goes to front brake lines and the inboard/back reservoir definitely goes to the rear brakes … however…

I did a look around at 80 masters parts diagrams and it does look to be reversed … front to back and back to front … as @h82crash said above

May want to have somebody confirm which line goes where on the 80 master

frt port frt brakes non abs 8x

 

[roma042987]

I've been busy with a couple of other projects while I've let this situation stew in the back of my mind. This evening I was looking at some other threads which reminded me I installed this same master on my 60 series in 2017. I checked the 60 a few minutes ago and what do you know....it was dark outside so I pulled this photo from my 60 thread.

Front port goes to the prop valve for the rear brakes.

 

[Skreddy]

I've been busy with a couple of other projects while I've let this situation stew in the back of my mind. This evening I was looking at some other threads which reminded me I installed this same master on my 60 series in 2017. I checked the 60 a few minutes ago and what do you know....it was dark outside so I pulled this photo from my 60 thread.

View attachment 3241674

Front port goes to the prop valve for the rear brakes.

Try swapping and see, I’m really curious to see if you notice any difference. I was adjusting my rear drums yesterday and was contemplating swapping the brake line ports just to see, but I didn’t have anyone handy to help bleed them afterward.