Loosing brake fluid

[asavage]

Originally Posted by aerocolorado

Only once or twice have I felt the RABS come on during slick roads or panic stop. The panic stop elicited a large, strong, intermittent pedal sensation quite unlike the rapid, small pulsations on newer vehicles.

We haven't had enough ice/snow here yet for me to functionally test the RABS on my "new" '93, but looking at the manual, there's no speed sensor for the front wheels, so the RABS is looking at the decel rate of the VSS signal at the rear diff only, which seems like a poor way to figure out if the RABS should be activated or not. Or am I missing something?

[xlt4wd90]

I remember those days in Detroit when I was laying in the snow trying change a starter, or installing a floor shift kit. I don't want to do that again.

On the other hand, I once helped a friend work on his car in the dead of winter. He had one of those pop-up tarp shelters, and ran a gas heater inside. It was quite tolerable even when the temps outside were in the minus-teens. Just a thought.

[aerocolorado]

Originally Posted by asavage

We haven't had enough ice/snow here yet for me to functionally test the RABS on my "new" '93, but looking at the manual, there's no speed sensor for the front wheels, so the RABS is looking at the decel rate of the VSS signal at the rear diff only, which seems like a poor way to figure out if the RABS should be activated or not. Or am I missing something?

That's correct, rear only. RABS = Rear Antilock Brake System. The RABS system was Ford's attempt to account for the difference between an empty and full loaded truck. Chevy and others use an adjustable proportioning valve to distribute braking forces.

[Aeroman59]

Originally Posted by aerocolorado

That's correct, rear only. RABS = Rear Antilock Brake System. The RABS system was Ford's attempt to account for the difference between an empty and full loaded truck. Chevy and others use an adjustable proportioning valve to distribute braking forces.

So what triggers the RABS? must be a combination of
VSS=0 (or too rapid deceleration) & ......BOO switch= On

I'm having a bit of a problem with the way it works since, there is only one master cylinder & it looks like there should be independant hydraulic control of the front/rear brake lines! or is this achieved solely by timed piston displacement within the RABS unit ?

[asavage]

Originally Posted by Aeroman59

So what triggers the RABS? must be a combination of
VSS=0 (or too rapid deceleration) & ......BOO switch= On

Here, I quote from the Ford CD:

Originally Posted by Ford CD

[ . . . ]When the brake pedal is applied, the RABS II module senses the drop in rear wheel speed. If the rate of deceleration is too great, indicating that wheel lockup is going to occur, the RABS II module activates the electro-hydraulic valve causing the isolation valve to close. With the isolation valve closed, the rear wheel cylinders are isolated from the master cylinder and the rear brake pressure cannot increase. If the rate of deceleration is still too great, the RABS II module will energize the dump solenoid with a series of rapid pulses to bleed off rear wheel cylinder fluid into an accumulator built into the RABS valve. This will reduce the rear wheel cylinder pressure and allow the rear wheels to spin back up to vehicle speed. Continuing under RABS II module control, the dump and isolation solenoids will be pulsed in a manner that will keep the rear wheels rotating while still maintaining high levels of deceleration during braking.

At the end of the stop, when the operator releases the brake pedal, the isolation valve de-energizes and any fluid in the accumulator is returned to the master cylinder. Normal brake operation is resumed.

[ . . . ]The red BRAKE warning light is constantly monitored by the RABS II module. If the red light illuminates, the module will disable the RABS II system. For this reason, diagnosing and resolving any base brake concerns illuminating the red BRAKE warning light will, in most cases, resolve a perceived RABS II concern.

Of note (to me) is that if the brake fluid level is low or the pressure differential switch grounds (air in one brake circuit) and the red BRAKE lamp comes on, that deactivates the RABS.

Looking at the RABS wiring diagram, the Parking brake condition is NOT monitored by the RABS, even though the Parking brake switch lights the red BRAKE lamp. This is through the "Brake Warning Resistor/Diode Assy.".

The RABS module does monitor the BOO.

Now you know everything I know about this antiquated RABS. Not too much.

Originally Posted by Aeroman59

I'm having a bit of a problem with the way it works since, there is only one master cylinder & it looks like there should be independant hydraulic control of the front/rear brake lines!

The single master cylinder is actually two more-or-less independent hydraulic units. There are two steel lines coming out of it: one for front disks (no residual pressure valve), one for rear drums (contains residual pressure valve). From memory, both lines go to a pressure differential sensing valve (with a switch to light the red BRAKE lamp), then the line to the rears goes to the RABS block for modulation before heading to the back wheel cylinders . . . oh, all right, I'll go out and look . . . yup, just like that.

[Aeroman59]

AT LAST. Yesterday I put on another Combination valve, due to the aforementioned leak. Very tricky to get the tube nuts on, tendency to cross thread, however everything went on with no other problems so far! Today I have to bleed the system. Pedal is spongy, but no lights on, I do have brakes................ anything to look out for ?

Regards Aeroman.

[Aeroman59]

Well I certainly seem to be having some trouble getting the air out. Could not bleed anything at first since the bleed screws were blocked. Going round from rear to front a second time. I had a hard pedal at first then lost it. Air in the RABS unit maybe?
I have heard there is a bleed unit which holds open the RABS valve to get out the air, is it really needed?

Don't have any leaks that I can see.

Aeroman.

[VanGo]

I've had difficulty with bleeding the STS ABS system in the past. I finally took it in to my trusty (only last resort) mechanic and had the system flushed with new fluid. Sponginess was gone for good. And everything is fresh.