PV-Entusiasten - editorial

Disc brakes on the PV

English translation of the story
Sätt skivor på PV:n
in the PV-Entusisten # 2 1999 page 34-35.

By Dan Jansson

One modification that many express interest in knowing more about is the installation of disc brakes on the PV or Duett. As always, when it comes to brake jobs, it is important to do things the right way. Furthermore, the steering system can be affected if the modification is not done in the correct way.
When the PV was introduced, the brakes were claimed to be among the best in the industry. The American manufacturer Lockheed supplied the equipment and these same brakes were fitted on many heavier American cars. Thus there were plenty of good reasons to believe that the brake performance would be excellent.
When the P 544 was introduced in 1958 both the PV and the Duett had brakes of the Duo Servo type. This means that the two brake shoes have a floating connection in their lower ends giving an increased brake power. Despite this, the brakes still had all the drawbacks that are associated with drums including the condition referred to as "Brake Fade" which typically occurs during episodes involving extreme wetness or heavy usage which developed high amounts of heat.
Maybe it is not so surprising that the brakes were claimed to be efficient in the late 1940's considering the speeds that were used by then. But one who has put a beefed up engine in his PV and intends to make use of it‘s potential will also wish to ensure that he is able get the vehicle to a stop in an assured manner. The safest way then is to modify the brake system with disc brakes and servo assistance.
Already in the 1960's, rally-equipped PV's were modified with disc brakes and during the 70's and 80's, when the PV and Duett were popular for "hot rodding", a lot of people wanted discs. The Swedish magazine "Start and Speed" in the 70's produced a series of articles called "Project Duett" in which a Duett was modified in a way that was typical for that time. The fenders were extended, the side windows taken away, VW K70 rear lights fitted and a custom front made. An overdrive gearbox and a new dashboard and on top of everything, a Rover V8 engine was also included in the plans. ...And of course, disc brakes.
These were probably absolutely necessary in order to get the car certified with the more powerful and heavier V8 engine.
There are arises several problems when disc brakes are fitted on the car and there are differing opinions on how to solve these problems.
The shock absorber can not remain in its position aft of the control arm. The brake caliper would then make the turning diameter way too big. When the shock absorber instead is moved to a position forward of the control arm it instead interferes with the steering arm, also giving an unacceptable turning ratio. The lower shock absorber attachment therefore has to be modified. This can be made in different ways and is detailed later in this article.
The most suitable combination of parts is a complete brake system from a Volvo 122 with disc brakes on the forward wheels, drums in rear and the reduction valve on the brake line to the rear axle. Brakes from cars made in 1966 – 1968 are to be preferred. Also brakes from 1965 can be used but they do not include the reduction valve for the rear brakes. Power brake assistance is recommended – this can also be taken from the 122. The power brake system should be hydraulic operated and attached to the main brake line from the main brake cylinder of the PV.
Some important items when fitting disc brakes on the PV:

The bolt holes on the caliper holder have to be adjusted to fit with the holes in the kingpin holder. One hole fits "almost exactly" and has to be expanded just a little so the caliper holder can be centered on the wheel axle. Then the three other holes can be indicated and drilled.
The holes on the protecting plate behind the disc must to be modified in the same way as the caliper holder. The brake tube holder on the back of the protection plate has to be modified for brake line clearance.
The brake disc fits on the PV steering knuckle. 120's and PV's share the same front wheel bearings.
The shock absorber that is normally aft of the support arm has to be moved forward to provide enough space for the caliper. On the upper support arm the attachment can easily be turned the other way around. On the lower arm a new attachment bolt has to be fabricated. One way is to use two original attachment bolts and weld them together with an axial offset.

Another way is to make a cut in the bolt, bend it and fill the gap with the welder. However this requires a very high quality welding as the bolt is exposed to heavy forces and vibrations. (See above picture).
One of the two is to be cut off just outside hexagonal end, the other one just inside. The two are welded together with the hexagon ends towards each other. The new attachment is then mounted with the part where the shock absorber is to be attached upwards. Test with the shock absorber attached for to insure no interference of the steering arm.
The reverse valve in the brake master cylinder has to be removed. It's function is to keep a certain pressure in the brake system. That pressure being balanced by the return springs in the drum brakes system. In the disc brake system the pressure can not be there as the brakes then will be engaged.
The rear brakes should preferably be changed. The original rear brakes on the PV 544 are Duo Servo. With front disc brakes the rear brakes are likely to lock up prematurely when applying brake power, even if a reduction valve has been fitted. Rear brakes from the 122 are recommended.
Power brake assistance is recommended. With no servo-assist, the brakes will likely need a lot of pedal pressure.
If a brake servo is fitted, the three-way junction on the master cylinder has to be removed and replaced with the four line junction which will also permit fitting the brake light switch. This can preferably be put to front end of the master cylinder attachment. The original brake lines and electric lines for the brake lights can then be used.

These guidelines are very workable in theory and in actual practice. Absolute care and caution should be exercised when performing modifications to any brake system. While endeavoring to enhance your PV or Duett's ability to stop with greater efficiency is an worthwhile goal, even the slightest oversight or error (yours or ours) may put you and those around you at great risk.
Never "cut corners" or take "short-cuts.
It is prudent to have your brakes inspected by a professional once the project is completed.

Original story and pictures by Dan Jansson,
Translation by Dan Jansson (thanks to Mark Hershoren for assistance)

Disclaimer:
The modifications detailed herein are performed solely at the risk of the person doing the work. The Svenska PV-klubben will not assume responsibility for the end results or possible damages that could result from undertaking modifications to any automobile braking system.