yellowtoytruck

Hey guys. I am needing some technical / trouble shooting help.

I recently upgraded my 1985 Toyota Pickups brake system and now I am having issues.

What I upgraded:
1. 88’ Rear axle with the bigger drum brakes.
2. master cylinder to a big more master cylinder from an FJ80 from marlin crawler
3. disc brake kit from FROR for Tacoma disk and calipers up front.
4. Dual Diaphragm booster from a T100 brand new

What is happening:
My rear drum brakes are locking up when applying pedal. I can slowly depress and get to a stop without lock up. But if I was any occasion I had to stop quickly I would skid. I feel like my front brakes aren’t getting as much hydraulic pressure as they should.

I also feel that my brake pedal isn’t as firm as it should be. But since I replaced pretty much everything in front of the pedal I don’t know if this is the new norm. The pedal is firm with the truck off. When I turn the truck on it becomes very soft.

What I’ve done
1. Bled Master Cylinder again on the bench. No bubbles came out and I continued to bench bleed long after I discovered this just to make sure.
2. Bled brakes again. No air is coming out of lines.
3. Verified brake booster check valve is pointing the correct way ( towards engine )
4. I also went through and verified all the brakes lines are tight
5. Disconnected the LSPV and lowered all the way down simulating no load. Still happened.
6. Adjusted the rear drums to loosen them. Used the star adjuster and put a screw driver behind it to turn it backwards. Still happened.


What do you guys suggest my next step be? I still have the stock LSPV. With these upgrades should I eliminate it? Any thoughts and ideas would be appreciated.

Damion812

I myself have just got into this LSPV thing but not new to brakes. Although most vehicles usually have some sort of proportioning valve 70 front 30 percent rear. But it almost sounds like you have the LSPV out of the equation even when it's hooked up . So too much braking power is going to the rear. I researched what the LSPV does and found some good info on how people eliminated it with an adjustable proportioning valve from Summit. Maybe that's what's needed. Oh yeah my disclaimer here: I'm no expert mechanic or anything like that just a do it ur self type for 30 years😂

akwheeler

If you are sure that all of the air is out of the system it is probably that your new brake combination is not all designed to work together, you will probably have to put in an adjustable proportioning valve.

Don't forget there is a brake line connecting the LSPV and the front brakes that allows the rear brakes to get full power if you lose hydraulic pressure to the front brakes.

things that can cause your rear brakes to lock: leaking rear axle seals, improperly adjusted parking brake, air in the front brake lines, seized front calipers, pinched front brake lines or bad hoses, broken return springs for the rear shoes, improper assembly or bad parts in the rear.

there are probably some causes that I have missed or forgotten, that's all I got for now.

dropzone

iTrader: (6)

Did you also bleed the LSPV?
Did you replace the wheel cylinders on the rear axle or anything back there?
New or used calipers up front?

Most of the time on a conversion like you have done it is a good idea to bypass the LSPV and use an adjustable proportional valve like this
https://frontrangeoffroadfab.com/pro...ing-valve-kit/

you might consider giving Brian a call at FROR or email him since it sounds like you got most of the stuff from him and run it by him

yellowtoytruck

dropzone

iTrader: (6)

good deal that you got it figure out and updated your thread.
what you found totally makes sense

akwheeler

Yup, that'll do it.

Damion812

Good news right there!

thefishguy77

iTrader: (1)

Well done and thank you for listing the fix.