My name is Tom, I live in upstate New York,about 60 miles from Albany,and am glad to meet you all.I am a Volunteer Firefighter of about 10 years,and own a 1988 Toyota Pickup 4x4 with a V6 and auto tranny.I have owned the truck for about two years,and I bought it from my mom after my dad had passed away.my dad bought the truck new,and (Yes I have already replaced the head gaskets) I use the truck to plow snow.I have recently been having a real bad issue with the brakes on the truck,and I am getting real frustrated with it.I have been noticing that for about the last month or so the brake pedal was getting lower and lower and pumping the brakes does not help.I replaced the Master cylinder,and booster and still no difference.It does not have any leaks and if you pump the brakes with the engine off,after about 2-3 pumps it has good pedal and it will hold as long as you dont let off the pedal.If you let off the pedal and step back on it,it goes to the floor.Start the truck drive it to an empty parking lot,and the pedal just plain gone,no matter how much you pump it. I have bled this thing a million times and am getting no air from any bleeders,just good clean fluid.I also bled the Leveling valve and no air there either.Am i missing something here?

 

Did you bench bleed the new master cylinder prior to installing it? What sequence are you bleeding the brakes and LSPV in? Do you have good solid vacuum to the brake booster?

 

My experience with the brake booster, when it goes bad the pedal is hard but you can put enough pressure in the system to apply the brakes (just like when the engine is off). If everything is bled properly, my guess would be the rear brakes are WAY out of adjustment. Pull the rear wheels, make sure AGAIN that the wheel cylinders and axle seals are not leaking. Then, adjust the rear brakes so they put a little drag on the wheels when you turn them.

See if that solves the problem.

 

Check the hoses as well, you could have a small vacuum leak.

 

Ok I re-bled the system (again) left rear then right rear left front right front then the LSPV last.I have bled this thing fifteen ways to China and no difference.Check what Hoses for a vacuum leak?The vacuum to the booster is good.I have NEVER ever had a vehicle stump me like this,I really thought it was the Master cylinder or booster.Thanks again for the suggestions and as they come in I am trying things.I am going out in the garage now to check the rear brake adjustments.If it is a wheel cylinder that's gone bad wouldn't I be using fluid?

 

Have you tried bleeding with the engine running? Had same problem as you trying to bleed with engine (not) running. Grabbed a helper started engine bleed each wheel and lspv brakes came back. This was on a 1988 toyota 4x4

 

Try taking a wrench and tap (don't pound) around on the Master Cyl. housing. Pump the brakes, tap around some more. You might have air still trapped in the MC like Roger suggested. I had that problem and after a bunch of tapping and bleeding the brakes came back to me.

Here is a link to the FSM Brakes section for a 93 truck but it should give you, a good idea how to check over and adjust/bleed everything including the MC while on the truck.

93 FSM

 

Bump...What happened?

 

Tom,
In my opinion you need to create a diagnostic fixture. You are dealing with hydraulics so your objective will be to isolate which component in your hydraulic brake system is at fault. Lets start by blocking the wheels so the truck can not roll. Next, look at the master cyclinder so you can make "U" shaped brake line which will connect to the master cylinder. You can purchase a small piece of steel brake line at any auto parts store for about $10 or so. Put a pan below the master cylinder to catch the fluid. Disconnect the two steel brake lines that attach to the master cylinder. Connect your "U" shaped piece to the master cylinder. Blead the "U" shaped piece by looseing the fitting at the master cylinder. Then test the master cylinder by sitting in the truck and pressing the brake. You should get a nice firm pedal and it should not go to the floor. Hold the brake pressure and see if it stays and does not bleed down. If it bleads down, the master cylinder is bad. Please tell me if the master cylinder you put in is an after market one or a Toyota one.
Thank You,
John N.

 

i had a similer problem cheaked every thing and finaly found out that the new master i bought was sticking and not returning like it should the fronts worked like they should but the rear did not when i opened the bleader on the rear i would get nothing i replaced it with a new one and every thing is ok