TR6 Early vs newer pdwa for Tr6 1975.

[Michael_830]

While replacing brake lines, ran into learning about the Pdwa, pressure differential warning actuator. A past mechanic had ground two of the lines into it, so may need to replace it. Before I learned about varieties of early vs later pdwas, I bought one on Ebay, but does not appear to be the same as shown in archives and articles. The 2 openings on each side are different sizes, one hole much larger than the other. Is this not going to work for me? And I read some were metric threads and others imperial? Any thoughts are welcome. Thanks.

 

[Tybalt]

As complete units they will interchange early to late, late to early. There are three styles. The earliest one has a brass body and uses cup type seals on the internal floating piston. You will find that one on TR250 and very early TR6 cars. Next was a brass bodied unit that used o-rings as the seals for internal floating piston. Finally there was a steel bodied unit. I don't recall if any of the steel bodied units were used in production cars but do know that it was in the spare parts systems.

As for the different sized ports, they are there to differentiate between the front and rear brake circuits. Again IIRC, the smaller port is for the rear brake circuit and is the forward most on the PDWA while the larger port is for the front brake circuit and is the rearward most set of ports on the PDWA. Since both circuits use the same size hydraulic line, that larger sized fitting is rather unusual. I wound up having to get mine from a local industrial hydraulic supply house that carried all sort of weird stuff in addition to the normal stuff you can find readily.

Threads are unified (interchangable with SAE) and not metric on any of the PDWA units referenced above. The only metric stuff used during production that I can think of for these cars in the brake system are the hardware for mounting the 16M caliper and the hard line that connects to the 16M calipers that would be fitted to the later (like your '75) cars. While original brake servos are unified threads, replacement brake servos from what I understand can have either metric or unified studs for mounting to the bulkhead and for mounting the brake master cylinder so if you replace your brake servo, double check those before installation. I happened to wind up with a replacement brake servo that was metric.

 

[Michael_830]

Tybault, Thanks for clearing up these pdwa issues for me. Just received brake lines for attaching to pdwa from TRF, and the different sized fittings/locations are just as you explained. Glad to know that pdwa bodies are interchangeable and I do not need to worry about metric on the pdwa. Though good to know about the metric on the servo. It's amazing how much confusion there tends to be with one small part. Thanks again.

 

[Tybalt]

Glad to help. I would like to clarify something from my earlier post though related to the bolts used for mounting the 16M calipers. The threads are Unified 7/16-20, same as the earlier caliper bolts. The important difference is that the smooth grip area of the bolts used with the 16M calipers is shouldered to account for the larger mounting hole in the 16M caliper. Also note I have seen these later calipers referred to as both 16M and M16P (it's a metric variation of the 16P caliper) although there is no reference to any of those designations in the old Girling or Lucas catalogs that I have, it's all by Girling part number even in the base casting cross reference listings.