Losing brake petal

Brake problem with our 2000 SR5 two wheel drive. Replaced front hardware then the master cylinder a couple of months ago to resolve a problem of the front brakes locking up. (No bench bleeding but did bleed it on the truck before hooking up any lines.) Since then a new problem has developed...losing brake petal. I have bled the breaks twice according to the book - RR, LR, RF and LF. May have let air get in by letting the fluid get low the last time when doing the RF, spent two hours bleeding that one cylinder and believe we got all the air out though (including the 'tiny bubbles'). Achieved great results with alot of petal and with a very firm petal. But, as the days went by we gradually lost petal, until now when it goes to the floor. Its as if we are sucking air into the system, which makes no sense at all to me. The brake fluid reservoir remains 'full'. I see no sign of a leak. The only fittings that have been disturbed are the brake lines at the master cylinder and the bleeder valves. Checked the master cylinder fittings and they sure seem tight to me. Does this sound familiar to anyone? Any ideas?

 

Did you bleed the LSPV near the spare tire?

 

No. What is a LSPV? I've never heard of it. Does it have a bleeder valve?

 

OK. The Load Sensing Proportioning Valve. From a web search I learned it shifts brake hydraulic pressure from the rear to the front wheels as you brake - as you brake the vehicle weight shifts towards the front wheels and without the 'pressure shift' the back wheels would tend to lock up / skid.

I found the valve just where you described and bled it. There was air in it. (I can see why with a big in-line body and located higher than the cylinders to boot. There is a picture of a rusted up one at
http://www.tacomaworld.com/forum/1st-gen-tacomas/137641-load-sensing-brake-proportioning-valve.html .) Not sure of the right sequance, so I bled the Proportioning Valve, bled the RR then the LR, then the Valve again figuring I was working back towards the master cylinder.

I have brake petal again. Hope it lasts.

Thank you, Rick (Tan 4x4)

 

You're welcome. I hope thats the final solution.

The LSPV also compensates for a heavy load in the bed, by shifting pressure to the rear. Actually, I've never had to bleed it in any of my rigs, but I've had to tweak it to compensate for a lift.

 

Have you checked your rear drums? And I don't mean....just the shoes. Check the cylinders & connections at the rear wheels.

If you keep getting air in the lines....you've got a leak. It sounds gradual since you had pressure after bleeding and it gradually lost pressure....

If you don't have any pressure currently, you will have to bleed the brakes again just to get pressure back so you can troubleshoot further.

Once you have pressure again..... with the truck in park and running, press your foot on the brake pedal as hard as you can and as far down as you can. Normally - the pedal will get to a certain point and stop. If your pedal continues to move downward even the slightest amount...you've got a leak. You just need to find it.

Good LUck