I recently replaced the rear crossmember (rusted out from years of living in Chicago) which involved removing & re-anchoring/resetting the proportioning valve spring, which over the years had become fragile. If the spring breaks, do I have to replace the entire valve assembly or is there a workaround? How critical is the proportioning valve to the functioning of the rear brakes? I remember reading somewhere, that it was designed to prevent the rear brakes from locking up.

 

No clear answer.

A replacement proportioning valve unit appears no longer available from Toyota and difficult or impossible to find.

If the spring is changed the valve may function differently. The valve can be calibrated but requires tapping into brake line pressures, setting the wheels on weighing scales and more.

The spring appears to be an override protector to prevent valve damage if the suspension arm moves too much. What appears to really matter is the rod adjustment. If a spring of the approx same length and compression force is used and rod adjustment remains the same, it may not affect operation that much. Just an opinion and what I would do if the current spring was too rusted to work. To prevent more rusting coat spring with LPS3 or similar.

The issue is to make sure the valve remains in working condition as replacements can be impossible to find.

Most brake systems have a proportioning valve. The rear and front brakes need to apply different brake line pressure to the brakes due to weight transfer and rear/front brakes may be a different design (disk or drum). The station wagon valve is more complicated as it actually reacts to suspension movement. This could be due to the possible different loading arrangement. The coupe and sedan have a contained unit that only reacts to brake line pressure.

 

I should have been more specific with respect to the "spring having sprung". I heard it pop as I was making a right turn at about 20 MPH. Attempts to retrieve it were to no avail; the proportioning valve spring is gone. Where do you suggest starting a search for a similar type spring? I did notice that it anchors to one of the lower control arms and extends up to the base of the proportioning valve and looks like it snaps into a proportioning valve bracket.

 

Why did the arm come off?

Don't recall how the valve assy is built. Would appear the spring should be able to compress more at some point perhaps when the chassis sways, otherwise why have one. Is one or the other arm end supposed to be free to move but is now rusted tight?

Measure the distance between the two points on the rod (or attachment bracket) where the spring would be installed. Then add some length for the spring to be in compression when attached. After that guess.

Assuming the spring is over travel safety device, it should be weak enough to compress if the chassis moves beyond the travel of the valve. But strong enough to keep the keep extended when the valve assy is installed.

You can get an idea of spring design, compression resistance, etc from springs at hardware stores. If the springs are to short but otherwise appears to do the job, take note of the wire diameter, spring outside diameter, coils per inch and guess at how many lbs it takes to compress it one inch. The shop the internet.

The link below has specifications on how springs are described

Utility Compression Springs by VALUE BRAND - Compression and Die Springs at Zoro

The spring "constant" relates to spring stiffness. The higher the constant valve, the stiffer the spring.

You just want a spring that does the job. Two springs stacked may work.