thewanderlustking

I have a 1985/6 1ton dually with an RV back, Mini Cruiser. The load sensing valve is set midway (I’ve never adjusted it), but the weigh of the back probably removes it from the equation, unless it is the problem…. The rv weight is 5500lbs. Currently running with it empty, nothing in the tanks.

The front brakes were upgraded From the 5 lug to the 6 lug setup from a 1991 V6. The calipers and rotors are bigger than the stocks were.

I have not yet changed out the master cylinder.

On my recent test drive after like 10-12 miles, the backs were hot and visibly smoking. The next day I lifted the rear thinking they were locked, nope they were easy to rotate.

Comparing the two Master cylinders, the stock might be a little smaller piston diameter, but overall it it like 2” longer. EDIT: Only 1" longer...

it is enough of a pain lift the rear end up that the simple thing, go drive it again, isn’t so simple.

Should I just go ahead and change the master out? I was a little worried about the difference in length.

I am also dealing with front brakes that needed some abuse to get cleaned up. My current master is fine, only need to change it would be to get the brake components all matched up.

Jimkola

I won't make any guesses on how the modifications you made changed your braking, nor will I hazard a guess as to a remedy. But here's what I can tell you about a regular Toyota pickup with a utility bed, flatbed, or RV.
What was common on these truck conversions was some sort of rear leaf stiffener added by the conversion co. to keep it from dropping in the back. Add-a-leafs, air bags, air shocks, etc. Some rigs had all of that. This pretty much negated Toyota's LSPV system, resulting in the front brakes doing virtually all the work and the rears practically nothing. About the only time we did brake shoes on a Toyota RV is when the inner axle seal blew out and diff oil saturated the brakes.

So it sounds like you modified or adjusted something that now has the rear brakes doing a lot. Personally, I'd stop with further modifications and back up till you figure out where things went wrong, but it's your call.

Question. Youi have six lug on the front, but five on the rear?

thewanderlustking

GOOD LORD NO LOL! I have 6 lug front and rear!!! I DO NOT HAVE the 5 lug "foolie" recalled rear axles of certain death. I have the 6 lug full floating rear, that came on it from Toyota and RBR (the RV coach/house builder). Yes though, I swapped the 6 lug fronts on from a 1991 (that had 6 lug dually wheels all the way around).

I have not added in extra leaf springs and it doesn't have any air bags. It looks like RBR might have added a leaf stiffener though. There is one attached to the swaybar mount that doesn't really look OE. It only has 32,000 miles on it and the 3 prior owners all kept receipts for even small repairs done. I don't think any of them would have done this, and not kept a record.

I have not messed with adjusting the LSPV. It is currently set midway and looks like it has been like that since it rolled off the assembly line, either Toyota's, or RBR's.

thewanderlustking

Thats why I am here asking! I couldn't wrap my brain around the combination I have, causing this issue.

I am not 100% sure I have an actual braking problem. Timing was WAY WAY over advanced and could account, perhaps, for the lack of vacuum assist. I did get on the brakes HARD a few times trying to clean up the fronts. Again, when I put the rears up on jack stands the following day expecting to find locked or dragging rear brakes, they turned easily.

Maybe in my case, I actually just have almost normally working rear brakes???

So all I have actually done to the braking system so far is:

1) Replace the front calipers with larger ones from a 1991 V6 Warrior RV (because I swapped the spindles to get the 6 lug fronts)

2) Bled the front brakes

(I know, not normal recommended DIY procedure, but you and I also know it is safely done in the shop all the time for various reasons, so let's ignore this lol. I mention it only in case there is some weird bleeding procedure that the LSPV adds to the mix. I looked and Chiltons didn't mention it, but saw mention elsewhere of it needing to be bled after all the cylinders and calipers are bled.)

What should I proceed with?

1) Do nothing, get it back on the ground and go test drive it again. Kinda my vote, and it might be the winner tomorrow unless a better suggestion is made before I do...

2) Flush the brakes. The fluid is somewhere between 200-250ppm. I have seen worse, but it is defiantly in need of a flush. Hence why I "don't think" I bled/flushed the backs.

3) Go ahead and put the matched 1991 master cylinder on there, bleed/flush the brakes, and then go test drive again.

4) Some other combo...???

thewanderlustking

I kept rewriting the above post, and am finally realizing that my confusion is geometry related... I think I am getting my brain wrapped around it now.

So the only major thing that changed from my earlier test drives, to the most recent one (aside from it being longer to better show a problem) is the larger front calipers.

It actually sounds like adding in the helper leaf spring would raise the effective height of the vehicle and artificially lower the weight. So if I am overworking the rears when unloaded, with the heavy house, instead of raising the bar at the axle adjustment point, I actually need to LOWER it... I think??? Lol

Okay what wasn't making sense was laziness prevails likely nobody (specifically RBR or the following owners) adjusted that valve, the above only works if somebody had. Until I factor in....

The larger volume on the front calipers, they are now moving less causing the back brakes to work harder. Hence the suddenly out of balance system.

I also suspect the longer length of the earlier master comes into play in reveres, but I don't know how much fluid each stage moves. This is just one more thing to crash the equation so we will have to ignore it. If they used a longer second stage to increase the rear brake pressure, more volume to the fronts would upset that even more. Theory only, but one that finally fits the scenario.

I just went and looked through the front wheels. The rotors cleaned up a little bit, but experience tells me they really should have been much cleaner. Defiantly have lower force on the fronts than it should.

So it seems the quick answer is, I can lower the LSPV bar a bit and probably rebalance the brakes. But the better answer is going to be to change out to the master cylinder that matches the larger front calipers FIRST. Then adjust the LSPV, if any is still needed.

I read through the currently posted FSM on the LSPV several times. If I could find specification charts to compensate for the heavier axle weights of the rv, it would actually be worth going through the trouble to correctly set the brake pressures and adjust the LSPV correctly. Otherwise it is kinda a guessing game....

Jimkola

The installation of the second gen brake calipers and rotors on first gen trucks has been discussed here before. Reading those threads would probably answer most of your questions. You're just most likely going to have to swag the info a bit for motorhome application. Especially when it comes to any sort of adjustment at the LSPV.

thewanderlustking

Jimkola, I didn't find any stickies. I did find some random bits, but nothing conclusive in all the threads I have been reading through. In fact they might have added more to the confusion than helped. If there is a thread you recommend, please? I am not wading through the mountains of Toyota info here well yet, but then a lot of it is still noise to me. Slowly building up my background knowledge to help make sense of it all lol!

I did find one tidbit. Seems that the '91 with the V6 came with a dual stage brake booster. Could possibly explain the difference in MC length. I need to study up more on that... Had to go to the local scrappy today. The RV I got the parts off was still there, but anything really useful, like the dual stage brake booster, was gone.

My momentary solution/plan is going to be to measure, take pictures, adjust the LSPV, and go for a test drive and see what happens. I grabded my laser temp gun from work again. I might even try a before/after. In fact I think that's exactly what I will do. If I can prove I am moving in the right direction...

G.I.G.O. My shakedown drive had some other things going on too, like the way over advanced timing. So with that stuff sorted out, I should go for another drive and get a better baseline. Toyotamotorhome.org has been down the whole week, so I probably need to try utilizing the wayback machine to read on the LSPV thread over there. There are some good testing and adjustment tips on there.

thewanderlustking

Alright update time! I have put a few runs on it and had varying results. Finally it acted up again for my friend and I and he got to experience it. The rear brake shoes are locking up. Especially in reverse. Most of the time going forward unlocks them, but not always and to varying degrees. I have it jacked up again and working on getting the axles out (stuck cone washers, of course) and inspecting. I will be doing a 4.88 rear end upgrade too when I figure out what will fit... That gets its own thread soon though.

Whatever the case, I think all my issues are simply partially frozen parts back there.