ABS not working

I have had this 1994 4Runner for 20 years. Many years back the ABS light came on and I changed the sensor and it has not come on since. However, the ABS does not seem to operate as I expect. To check it I used a snowy slick day. With the Runner doing about 10 or 15 MPH, I opened my door, stuck my head out so I could see the rear wheels and applied the brake. The front wheels lock as they should but the rear wheels lock as well. I performed the same test on dry pavement and this time the rear wheels did not lock, however the skid time period on the front wheels was very short since my speed was slow. The brake pedal has never pulsated as it does on my other ABS equipped cars. I have been trying to gather information on how this system works. I know it is somehow tied in with the power steering and I have been able to find most of the parts in the system on a parts diagram. But I can't find a system description anywhere. I have the maintenance manual but it is not much help in explaining the system.

Recently I changed out the power steering pump because it was leaking. I learned that after that the rear brakes needs to be bled with engine running and engine off. I did that but rear wheels still lock up on a slick surface. However, I don't know if I ran enough fluid thru to full satisfy the requirement. The brake pedal is firm so I don't think there is any air in the system.

Should the brake pedal pulse when the system is active? Which faults beside the wheel sensor will trigger the ABS light on? Any troubleshooting tips? Any ideas where I can find a system description?

 

You may have an issue with the LSPV valve (see pic). This proportioning valve prevents the rear wheels from locking up when you do a hard stop. It acts more like a pendulum than a computer controlled actuator so you don't feel the pulse. There is a lever arm going from this piece to your rear axle. If it is out of whack....

 

I found a consumer report article stating that on the 2nd Gen 4Runner the rear ABS works only when the vehicle is in 2 wheel drive. That's news to me and, after extensive searching, I have not seen that stated anywhere else. I posted above that I was looking out the door while applying the brake to see if it pulsed. However, I believe I was in 4 wheel drive.

 

That is correct. It only works in 2wd. Says so in the manual.

 

How is the ABS disabled when in 4 wheel drive? Is the rear wheel sensor disabled or what?