2007 Intermittent Brake Failure

[DMCEWAN]

Has anyone experienced an intermittent situation where the brake pedal goes all the way to the floor and the truck has no brakes? In my situation it happens immediately after turning the wheel all the way to the left or right for two applications of the brake pedal. The brakes then come back to working normal as long as you are not turning the wheel all the way.

 

[rbattelle]

Sounds like a problem with the hydroboost unit. Have you checked the power steering fluid level?

Ryan

 

[DMCEWAN]

Power steering level is fine, was hoping that was all it was

 

[rbattelle]

Well, then I suspect maybe a valving problem within the hydroboost unit itself. I don't think there's any servicing that can be done - you'd have to replace the entire unit as an assembly.

The service manual doesn't give any detail regarding the internal arrangement of the hydroboost. But I wonder if there's an internal pressure limiting valve that is sticking open when you crank the wheel against the stops and the power steering pump pressure goes way up. That might explain the problem you're experiencing, if the valve dumps back to the reservoir.

There is a bleeding procedure you might try, however, before going any further:

STANDARD PROCEDURE - BLEEDING
The hydraulic booster is generally self-bleeding, this procedure will normally bleed the air from the
booster. Normal driving and operation of the unit will remove any remaining trapped air.
(1) Fill power steering pump reservoir.
(2) Disconnect fuel shutdown relay and crank the engine for several seconds, Refer to Fuel System for
relay location and WARNING.
(3) Check fluid level and add if necessary.
(4) Connect fuel shutdown relay and start the engine.
(5) Turn the steering wheel slowly from lock to lock twice.
(6) Stop the engine and discharge the accumulator by depressing the brake pedal 5 times.
(7) Start the engine and turn the steering wheel slowly from lock to lock twice.
(8) Turn off the engine and check fluid level and add if necessary.
NOTE: If fluid foaming occurs, wait for foam to dissipate and repeat steps 7 and 8.

Ryan

 

[DMCEWAN]

Thank You!

 

[rbattelle]

If your power steering reservoir is already full, I would skip steps 1 through 4. [Our trucks don't have a fuel shutdown relay anyway].

Ryan

 

[brods]

Does this happen only while the steering is at full lock and you are holding the steering wheel to keep it from turning back some? If so I would suspect the relief valve in the steering circuit is allowing bypass (which it must or something would break) and somehow the hydroboost accumulator is draining down during this. Mind you this is all speculation until I can find a flow diagram for the power steering/hydroboost system.

Oops, Ryan already covered this.

 

[brods]

Tests for the 05 hydroboost (you'll have to find out if the 07 is different):



BOOSTER FUNCTION TEST

With the engine off depress the brake pedal several times to discharge the accumulator. Then depress the brake pedal using 40 lbs. of force and start the engine. The brake pedal should fall and then push back against your foot. This indicates the booster is operating properly.



ACCUMULATOR LEAKDOWN

1. Start the engine, apply the brakes and turn the steering wheel from lock to lock. This will ensure the accumulator is charged. Turn off the engine and let the vehicle sit for one hour. After one hour there should be at least two power assisted brake application with the engine off. If the system does not retain a charge the booster must be replaced.



2. With the engine off depress the brake pedal several times to discharge the accumulator. Grasp the accumulator and see if it wobbles or turns. If it does the accumulator has lost a gas charge and the booster must be replaced.

 

[sag2]

Check for clearance to the caliper in full lock. If you hit something solid the caliper piston can be pushed back in the bore and you will loose pedal until the piston can be pushed back to no clearance with a brake application or two.