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Also, on the subject of front disc brakes. I did this not terribly long ago on my PV. It took about an hour and a half per side, and much of the time spent was wrassling with stubborn stuck shock mounts.
First, you'll need donor parts from a 122 or 1800 - the disc, the caliper, the dust shield, and the caliper mount (bolts to the spindle with 4 smaller bolts, the caliper bolts to it with two larger bolts. Basically everything on the donor car's front wheel down to a bare spindle. On the PV, remove everything down to a bare spindle (drum, brake back plate).
I used the 'front shock' style. As normally placed, the shock's position behind the suspension arms will foul with the caliper. The shock mounts on bolts through the suspension arms which can be removed and put in 'backwards' so the shock sits in front of the arms. However, in this position there is a very slight interference problem with the bottom shock mount and the steering arm when turned full lock - solveable by cutting the bolt and rewelding it at a slight inward angle. On my bolts I just used a cutting wheel to cut most of the way through the thick hex portion of the bolt, bent the gap closed, did a little more grinding to make a nice channel, and welded the seam shut again. I'm no expert welder but they've stood up so far to about 1 1/2 years worth of spirited driving hooked to some Bilsteins.
With the shock out of the way, you next need to mount the dustshield (you will need to trim the brake line mount as it will hit the upper arm of the kingpin carrier) and caliper mount to the PV spindle. Just hold them up and you will find that one out of the 4 bolt holes is in the correct spot. You need to drill the dust shield and caliper mount for the other three (I think one of the other bolts holes is close, but not close enough). Once done bolt them up - it's probably a *really* good idea to use some heavy duty loctite and/or some new lock washers - you likely won't take them off again and you really don't want them coming loose.
Then put the disc hub on, the spindles are exactly the same and the disc fits perfectly. Bolt the caliper on, insert the pads. As for brakelines - probably the very best thing would be to use something similar to the 122 setup - with a short line coming from the caliper to the dust shield mount (drill a new hole on the shortened dust shield mount), then a flex line to the body mount. The as all standard sized components, and the styles of lines used are more common than the types used in later Volvos, so you should be able to piece something together at a good parts store. In my case I found that the original flex lines that attached to my drum cylinders would reach and fit the calipers. It takes a slightly circuitous route from in front of the wheel, past the shock (now in front of the suspension), past the kingpin carrier's upper arm, and onto the caliper. Despite the appearance, I haven't had any wear on the lines from rubbing against the shock or the arm - I wrapped tape around the line as an early indicator of wear and it hasn't shown any tet. I don't know if you would want to use the same technique or not.
A slight issue just might be present with the master cylinder. Way back when when cars had drums all the way around the master cylinder was engineered to hold a slight backpressure on the system. Only about 4 or 5 psi (less? I dunno really), but it was counter acted by the springs in the drums retracting the shoes, and served to keep the slack out of the system so the pedal would have a firmer, quicker feel to it. But disc brakes have no springs retracting the pads, the slightest backpressure on them will make the pads drag. The backpressure was held by the little gizmo in the master cylinder on the opposite side of the spring from the piston. The good news is that it is part of a rebuild kit, and rebuild kits made in the last several decades have included gizmos that don't hold backpressure (they just restrict the returning fluid slightly - otherwise you wouldn't be able to 'pump up' the brakes in an emergency). So unless your master cylinder has been untouched for decades (unlikely) or was rebuilt using a vintage rebuild kit (more likely) you won't have a problem. And if you do have a problem (characterized by dragging pads - sticky pistons also cause that but don't respond to a quick opening of the bleeder valve) a quick rebuild using a new kit will solve it.
And now that you've spent all that money (OK, not that much really - I got the parts for under $100 on eBay) and you go out and test drive. And you put on the brakes and .. .... well, nothing really. Under normal use they work exactly the same, perhaps slightly less powerful than the drums were. I did take the precaution with mine of finding a deserted, remote, empty stretch of blacktop to satify my curiosity about the new brake balance - to make sure the rears weren't now locking before the fronts in a panic stop (OK on mine, you mileage may vary). However, do several hard stops in a row and they no longer fade like the drums did. Which is why I did the swap in the first place - not a good sensation when you have to press twice as hard on the pedal to get some braking done.
And I must admit, there is a certain aesthetic benefit to the swap as well if you have wheels that show the brakes. The disc and caliper look much nicer to my eye than a puny looking drum. Costwise, it must be said that while drums and discs cost roughly the same, as do pads and shoes, new/rebuilt calipers are much more expensive than new/rebuilt wheel cylinders (also used on some 60's American cars and often available in local parts stores for a very nice price).
I know of one guy who also put discs on the PV back axle using a similar method - removing the brake back plate, installing a caliper mount in its place. And separating the drum from the hub by removing the lug bolts, doing the same to a front 122 disc, and reassembling the disc onto the taper axle hub. This has a couple of issues. First the brake balance will most assuredly be off - he had also added an adjustable brake balancer system to make up for that. And no handbrake - he added a line lock on the rear wheels for that.
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I'm JohnMc, and I approved this message.
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