1973 Caprice steering question

[volkerc]

Wondering if the following situation is normal or not.:
With engine running I can't steer to the left or the right, when applying the brakes and car standing still! I can only steer with the foot off the brake. Is that how it's supposed to be? Or what could cause this?

[silicon212]

A number of things can cause that - from low engine vacuum to low engine RPM, low power steering fluid, worn or damaged power steering pump, loose P/S belt, wear in the steering gear, wide tires, etc.

[volkerc]

Originally Posted by silicon212

A number of things can cause that - from low engine vacuum to low engine RPM, low power steering fluid, worn or damaged power steering pump, loose P/S belt, wear in the steering gear, wide tires, etc.

well, we can rule out the tires, as they are stock width, the rpm does not change, i don't see how the engine vacuum could affect it, cause there is no connection to the steering and the engine rpm remains stable. power steering fluid level is ok. p/s belt can be ruled out. that leaves the pump or the gear. my guess would be the pump not strong enough,...? but why would it do that? only turn wheels without the brakes applied?

[Blue Bowtie]

There are two factors involved.

First, applying the brakes forces the brake power booster to assist you in applying force to the master cylinder. This force is the result of stored engine vacuum in the brake booster reservoir. If the brake booster has even a small problem with the control valving, applying the brake can create a demand for vacuum, which will lower the engine RPM. Whereas smaller leaks tend to increase RPM (if the carburetor is adjusted properly), larger leaks will tend to decrease engine RPM. The drop in engine RPM due to any vacuum leakage, plus the normal drop in RPM caused by the steering pump load when under pressure may be enough to lower the steering pump pressure to the point where there is no assist in turning the wheel.

One way to diagnose this is to remove the vacuum line from teh brake booster and cap it. Start the engine, hold he brake (with no power assist) and see if the steering will assist in turning the wheels. Another way to positively determine that is to connect a tachometer to the coil and adapt a pressure gauge to the output port of the pump, then try to simulate the condition. If the RPM does not drop even a little when the steering pump is loaded down, you may have a weak pump, and/or fluid in the steering system which has lost viscosity in the last 35 years.

The second factor is the design of the front suspension and steering. Since the wheels are turned on a knuckle, and the centerlines of the wheels are outboard of the center of the vertical axis about which the steering knuckle pivots, the wheels MUST be allowed to rotate slightly when steering. The fact that you are essentially locking the wheels with the brakes creates an increased load on the steering system, requiring enough force to either overcome the brake friction and force the wheels to rotate through the turn, or to force the tires to skid on the pavement instead of rotating to allow the steering to turn.

The fact that you may have slightly lower steering assist from a worn pump, worn steering gearbox, old and less viscous power steering fluid, lower engine RPM from a leaking booster, coupled with the increased force needed to turn the wheels with the brakes applied may cause your symptoms. The system may just be worn to the point of barely providing assist under normal circumstances, and can no longer provide assist under these more severe circumstances.

It might be a good idea to connect the tach anyway, and determine if the hot curb idle spped setting is correct. If it is low, that will only compound the problem.