Sticky brakes - brake servo question

[jaymemaurice]

Hey guys, So I was driving to work the other day, gently tapped the brakes slowing into a round about and they didn't spring back up, braking force remained on all wheels... tried to lift pedal with shoe... stuck. Pushed pedal down more, and stuck on even harder and further!!! FUU Slammed it into low drove around the corner off the road brakes applied (all smoking at this point). Shut the truck off tried lifting the pedal to no avail... took a taxi into work. After work got her rolling and drove it in the sand for a bit to try to duplicate the failure to no avail. So a couple questions about the servo assembly... Without me pulling this whole thing apart, does anyone have any idea on how this could happen?? Is the servo input push rod mechanically connected to the output push rod (preventing me from pulling the pedal back manually) ?! If the issue was something stuck in the master cylinder, I'd think I can still pull the pedal back up but brakes would still be applied... is that not a correct assumption? (note the truck holds braking very well, so not the typical master cylinder failure here) If it is a issue with a valve in the servo, should it not free my brakes up instantly by removing the vacuum (by removing the No Return Valve with the truck off (is this even physically possible while booster under remaining vacuum)) any other ideas? I took this truck into the desert often and wouldn't be shocked if sand has gotten somewhere it shouldn't... cleaned the crud off from behind the gaiter and will look at it more later. Anybody here know about the brake servo assembly?

[Savannah Buzz]

sticking caliper(s). Replace in pairs on axle involved. Will be too corroded to rebuild. Flush break system fluid all four wheel, will take two big bottles. If no smoke, ou can use infrared point and shoot themometer to measere disc temperature after short drive, one will be hotter than others. Friction and heat hold hands.

[jaymemaurice]

It is like the pedal stuck, not calipers. All 4 at once.

Now that they have released discs are cool to touch if I drive and not hit the brakes. Everything is functioning normally now. Calipers themselves seem fine like new.

[Savannah Buzz]

Perhaps some odd type of ABS failure, could remove ABS fuse in under hood fues box for diagnostic purposes, or master cylinder bore corroded, but seems like you would notice that over time. Sorry I missed the all four smoking when I looked at the first post.

From the RAVE manual it looks like the ABS is between the master cylinder and everything else. It has a couple of controls and modules, like the EBD, any codes? A special reader is used to deal with ABS codes usually.

From the RAVE:

General

The brakes consist of front and rear disc brakes operated by a diagonally split, dual circuit hydraulic system with
vacuum servo power assistance. The system incorporates the following control functions as standard on all models:

l

Anti-lock Brakes (ABS), to prevent road wheels locking during brake application.

l

Electronic Brake Distribution (EBD), to control distribution of hydraulic pressure between front and rear axles.
Replaces mechanical pressure limiting valve of previous systems.

l

Electronic Traction Control (ETC), to maintain even torque distribution to the road wheels.

l

Hill Descent Control (HDC), to provide controlled descent ability in off road conditions.

Hydraulic system schematic
1

Master cylinder/brake servo assembly

2

Brake pedal

3

ABS modulator

4

Rear brake

5

Front brake

6

Hydraulic circuit

a

Primary

b

Secondary
For normal brake operation, brake pedal movement is assisted by the brake servo assembly and transmitted to the
master cylinder assembly. The master cylinder assembly converts brake pedal movement to hydraulic pressure.
Primary and secondary brake pipe circuits supply the hydraulic pressure to the brakes via the ABS modulator: the
primary circuit supplies the front left and rear right brakes; the secondary circuit supplies the front right and rear left
brakes. Vacuum for the brake servo assembly is obtained from the engine inlet manifold (V8 models) or a vacuum
pump (diesel models), through a vacuum line and non return valve. To reduce operating noise, sleeving is installed
on some of the brake pipes in the engine compartment and the pipes are located in sprung pipe clips.
For all control functions, the ABS modulator regulates the hydraulic pressure to the brakes to control the speed of all
four wheels, either individually or in axle pairs. Operation of the ABS modulator is controlled by the Self Levelling and
Anti-lock Braking Systems (SLABS) ECU. The SLABS ECU also operates warning indications in the instrument pack

to provide the driver with status information on each function.

[Savannah Buzz]

more info on the brake servo from the RAVE, might indicate another problem could be vacuum related, like a leaky hose, or inside the unit.

Brake servo assembly

The brake servo assembly provides power assistance to reduce the pedal load when braking. If the brake servo
assembly fails, the hydraulic system still functions but will require greater brake pedal effort due to the lack of vacuum
assistance.
Two integral tie bolts attach the brake servo assembly to the pedal and bracket assembly on the engine bulkhead.
The master cylinder assembly is attached to the forward ends of the tie bolts.
The brake servo assembly consists of a circular housing which contains two diaphragms, a central plate, a control
valve assembly, input and output push rods and a filter. The input push rod is connected to the brake pedal. The output
push rod locates in the primary piston of the master cylinder. A protective gaiter is installed on the control valve
assembly where it extends from the rear of the housing. A non return valve, installed in a port in the front face of the
housing, is connected to a vacuum line from the engine.
The control valve assembly consists of a valve body containing a valve, a piston, a valve spring and an input rod
spring. The valve controls a vacuum port in the valve body. The piston controls an air inlet port between the valve and
the piston. A reaction disc and a ratio disc separate the piston from the output push rod. A guide tube on the front of
the valve body is attached to the front diaphragm and supported in a bush in the central plate. A return spring locates
in the open end of the guide tube.
The two diaphragms and the central plate separate the interior of the housing into four sealed chambers. The
chambers at the front of the diaphragms are connected together through fixed passages in the valve assembly. The
chambers at the rear of the diaphragms are connected together through the interior of minor diaphragms on the tie
bolts.

Brakes off

With the brake pedal released, the piston in the control valve assembly positions the valve so that the vacuum port is
open and the two pairs of chambers are connected together. When the engine is running air is evacuated through the
vacuum line and non return valve, creating a partial vacuum in all four chambers. When the engine stops, the non

return valve closes to maintain the partial vacuum and, on V8 models, prevent fuel vapour entering the brake servo.