Diagnosing brake issues / adjusting LSPV with pressure gauges (1 Viewer)

cartercd

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There have been many threads on adjusting the LSPV, ditching the LSPV, poorly operating brakes, etc. A great way to understand what is going on with your brakes is to measure the pressure at the calipers. This is the method prescribed in the FSM for adjusting the LSPV. In the absence of Toyota's specialty gauge set, I devised my own cheap method. Follow along for some OCD fun:

- Safety first. You'll be running the engine to take advantage of the brake booster and crawling under the truck, so make sure you are in a well-ventilated area, emergency brake engaged, and wheels chocked.

- You need two pressure gauges. The easy way is to buy the SSBC A1704 Brake Pressure Gauge Kit. You get 1 gauge for $50 with 6 adapters for a variety of brake fittings. So $100 for a set, available from Amazon, Jegs, Summit, Ebay, and others.

- I never take the easy way, I take the cheap way. I made my own gauge set.
- Winters PEM Series Steel Dual Scale Economical All Purpose Pressure Gauge with Brass Internals, 0-3000 psi/kpa, 2-1/2" Dial Display, +/-3-2-3% Accuracy, 1/4" NPT Bottom Mount. $15/each on Amazon.
- When you screw this into the rear caliper the 2.5" diameter gauge comes close to hitting the tire on each revolution. No problem for my pizza cutter 255s, but with bigger tires I would find a 1.5" gauge. The SSBC gauge above is 2". You could also rig up a 90 degree elbow or flex hose to avoid clearance issues.

- You need to adapt the NPT threaded gauge so it screws into the 10mm caliper fitting. Auto Meter 2265 Metric Adapter/Oil Pressure fitting does the trick. $11 each on Amazon. It did require a trip to Home Depot for some miscellaneous fittings. The AutoMeter adapter uses a copper crush washer to seat to the caliper face - it isn't a bad idea to have some extras available.

Here is the gauge setup.

Brake gage.jpg

brake gauges set.jpg


The FSM calls for setting the rear axle load at 2,932 pounds to check LSPV. I went to the scales at a local moving company and came in it at 2,700 pounds front and 2,600 pound rear (51% front, 49% rear). If you don't ask for certified weight tickets they might do it for free. I have lightweight front bumper, sliders, Kaymar bumper without swingout, 255/85 Cooper tires, 2nd and 3rd row seats removed, all tools removed, 10 gallons of fuel. Although heavier than stock, the front/rear weight distribution is close to stock. I then took a bathroom scale and loaded 332 pounds of stuff to the rear to get me to 2,932 pounds, achieving 48% front / 52% rear weight.

Next you need a way to hold the brake pedal in a fixed position while you take readings. I was doing this solo, so I rigged up a pipe clamp. You could have a helper press the brake pedal, but there is risk that there could be slight foot movement between readings.
Brake holder.jpg


I installed the gauges on the left side calipers (sorry, deleted the pic), started the engine, pumped the brakes several times, and locked the brake pedal down with my pipe clamp. I then read the gauges. The FSM says read the gauges within 2 seconds of each other - good luck with that. Note that when you screw in the pressure gauges until the crush washers seat, the chance of the gauge faces being in a direction you can read are slim to none. I have a $20 borescope that hooks to my cell phone that allowed me to hold the flexible camera on the gauge and I could see the image on my phone. Worked great. Inspection mirror is another option. I then applied more brake pressure and took the reading again, logging my results. I repeated this whole process a few times until I collected 20 data points. Unless you have some type of ratcheting brake adjustment mechanism, you are lucky to be able to hit a pressure within 50 psi of your target. You need to try to hit 1,138 psi to see if you are in spec, and ideally do this a few times.

Below are my results.

- Orange line is what would happen if there were no proportioning valve: 50% front / 50% rear pressure
- Purple line is OEM spec for LSPV that is in uppermost position (no load sensed), which maximizes from pressure
- Blue line is OEM spec for LSPV when rear axle is loaded at 2,932 pounds and front pressure is 1,138 psi. This is rear pressure of 869 +/- 86 psi
- Green dots are my recordings, with the green line being the best fit curve courtesy of Excel.

Brake pressure for LSPV.JPG

Around 1,138psi front pressure my rear pressure is within spec. Furthermore, the LSPV appears to be doing its job. At low pressures I have 52% / 48% brake pressurebias, at the spec pressure I am at 55% / 45%, and at high pressure I am at 62% / 38% (the harder you brake, weight shifts to the front axle, and brake proportioning shifts to the front axle). I also removed the 382 extra pounds I loaded in the rear and took another 5 readings. As expected, the LSPV provided around 5% more bias to the front axle. I did not have to make any LSPV adjustments, but the FSM has directions on how to do this.

After all was done I bled my brakes. Although removing the bleeder screws doesn't introduce much air into the system, better safe than sorry.

Caveats:
- Aftermarket springs will alter how much the LSPV moves relative to the OEM springs. So the LSPV will not perform as designed, but may be better than no LSPV at all.
- Heavy bumpers, winches, and dual batteries will also change the theoretical optimal brake bias, so the OEM LSPV may not be ideal for these conditions

Cheers:beer:
 
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Joined
Feb 11, 2011
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Wisconsin
this is what is use to set my brakes
screen shot windows

you can get kit like this for used off ebay for 70 $
some people just will tell you just turn 2-3 turns well it does not work that way just remember you r working on brakes
after any lift you must adjust that valve is you do 4" lift you will get about 450 PSI on the rear and you need about 850 PSI
 

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