strange brake issue .. only getting 50% brakes

Started having an odd problem with the brakes a few days ago, that I plan on fixing within the next 24 hours, but don't understand what is wrong.

If you push the brake pedal like normal, it doesn't matter how hard you push the pedal you only get about 50% of the stopping power that you should get. However it you then release the pedal a little bit (not the full way) and then push the pedal again, you get 100% stopping power.

It doesn't matter how hard you push the pedal, you can not get the brakes to work any better, and I even attempted to get the wheels to lock up, and couldn't.

I plan on getting someone to ride with me and take it to a gravel lot, and see if we can get the wheels to lock up, since I think only the front or back brakes are working.. I would like to know what wheels are not engaging.

Why would it only have 50% stopping power, unless you double pedal the brake pedal?

 

Just did a brake check on gravel, and the front wheels locked up..

 

1998 Toyota Tacoma, Extra Cab, 4x4 310,000 miles


I think its a value problem of some sort. I feel that if only the rear brakes were not working, that I would have a different feel at the pedal, and have better stopping power. It's almost like the rears are getting zero pressure, and the fronts are only getting 50-60% pressure, until you hit the pedal a second time..

 

My only guess is air in the brake line. Air volume goes down as pressure goes up, making a portion of your pedal travel only compress the air rather than pushing on the brakes. If you pump the pedal you can compress the air far enough that you get effective brakes again.

However, if you haven't had the brake lines open you can only get air in the system through a leak. If it were me I'd start pulling wheels and looking at the brakes for a leak. That is, that's what I'd do until someone that knows more than me answered my question.

Let's see if you get a better answer than mine. I'm betting you will.

 

Maybe pull the rear drums and see if any shoe is left. I'd start there. Another possibility is the rear proportioning valve may be stuck closed.

 

Check vacuum lines to the brake assist for leaks too.

 

Does the pedal feel normal? or too soft/hard?
I'd start with a basic brake inspection...
-Vacuum lines to the booster
-Leaks at or around Master cylinder
-Leaks near calipers/ wheel cylinders
-Pad/Shoe material left
If everything looks good, do a proper brake bleed. By bleeding the rears you'll be able to tell if the LSPV is stuck closed.

 

there is a valve on the back brakes that turn them off until you have a load in the bed.

its on the frame rail connected to that long rod on top of the rear axle.

as the bed presses down it opens more brake pressure to the rear brakes, as you empty the bed it closes and shuts off pressure to the back brakes. this is designed so you don't lock up the rear brakes and have the truck spin around on you when braking hard.

I don't know the specifics but its like 70% front brakes and 30% back brakes or something.

if this valve is bad or adjusted wrong then you might have 0% rear brakes, if you haven't lifted or lowered the truck it should be set correctly but my bet is it is out of adjustment or some idiot didn't know what it was and removed the brake setting arm from it completely

 

I took the truck into the shop, and they looked at the brakes. They told me the rears were out of adjustment.

I need to do a bit more test driving, however I think that it still is not working correctly.

I think that "keakar" is on to something, I have had a set of 33" wheels and tires in the bed of the truck for the past few weeks, changing the load on the truck. Maybe when I hit the brakes the first time it allows the value to move to a different setting, then when I immediately hit the brakes again, the setting has changed, making the brakes feel different.

I will get some test driving in today, and possibly remove the extra weight from the truck and see if that helps.

THANKS for the ideas.. I will post what fixes the problem once it's fixed.