The correct radiator cap for an SM is 4 PSI. If you have one with a higher rating then if the water circuit is pressurised from a fault, the seal may possibly leak. So first fix the fault. A SM with a good working radiator does not barely pressurise the water system while running, except possibly in the United States desert southwest or other locations where the ambient temperature is above 40C or so.
Original water pump seals on the SM are reasonably reliable, however, they do have several drawbacks. The first is that the seal runs directly on the back of the bronze impeller. That means as the seal wears, it scores the surface of the impeller itself, which then must be replaced, machined back or otherwise repaired. The seal itself was made by a (now defunct) company called George Angus and Company (abbreviated GACO) founded in Newcastle, United Kingdom. The proper seals have an identification mark "GACO WPF087." Similar seals, with an inferior spring (identifiable by the marking "GACO CF28783") found their way into the parts supply chain in the late 80's leading to significant reliability issues.
For more information see Media:Citroenthusiast_Article_on_CF28783_seals_ABX.pdf
There is no short term problem if a correct new seal (marked "GACO WPF087") is fitted, but it may be best to modify the installation slightly for a long term solution. The spring inside the original seal was stiffened in production, and a design shortcoming in the engine can mean that on certain engines this later spring will go 'coil bound' putting a significant pressure on the seal face. Ultimately the running face in the turbine will wear unevenly, also the spring does not press on the seal rubber to carbon face a full 360 deg, and the area not loaded can de-bond leading to a leak.
Mk2 seal incorporated a modified timing cover (seal seat is smaller diameter), jackshaft (impeller pilot diameter is smaller than MKI) and new design impeller (Aluminium alloy with ceramic insert). Fitted to later & some factory reconditioned engines and most Meraks. The seal itself (but not the ceramic seat) can be sourced easily as it is nothing more than a PacSeal 600 (5/8 inch type 68) industrial seal. These do eventually leak if anti-corrosion in the coolant is inadequate, the ceramic gets lifted from the turbine by corrosion. You can usually fix this by cleaning the turbine, painting the recess with a good resistant paint and re-fitting the ceramic and its rubber mounting using silicone gasket.
MK1 water pumps are easily distinguishable from MK2 water pumps in that all MK1 water pumps have a bronze impeller whereas all MK2 water pumps were fitted from the factory with an aluminium impeller and ceramic running surface.
Consists of either of - 'washer' for 360 deg contact and ceramic; or new seal with 360 deg washer fitted and shortened spring to avoid coil binding, plus turbine with ceramic (on exchange).
Don James (JBM, Kent Ohio) supplied a seal with a stiffened spring in the mid 1990's. The spring was made from a vanadium steel alloy rather than stainless steel. It was later discovered, however, that although the vanadium steel alloy spring was stronger than the original spring, it suffered premature corrosion failures. If you have a JBM seal you can retrofit the spring from an OE seal to create essentially a reproduction seal (slightly weaker spring but one that will not fail from corrosion).
A water pump modification that uses a PacSeal 601 (3/4 inch type 68) seal in place of the expensive and unreliable MKI water pump seals. The PacSeal 601 type seal easily seals well over 7 psi. so you can run the cooling system at higher pressures to suppress localized boiling. You can even use a coolant recovery cap (which are available only in 7 psi and up). Unfortunately, this modification can only be done when the engine is out for overhaul or some other work because you need to trim one surface on the jackshaft slightly (i.e. in a lathe). Once the mod is done, you can still go back to the MKI seal if you ever want to.
A MK1 seal is approximately €50 (Jan 2015 price) from SM Club de France, but is nearly $300 (Jan 2015 price) from the commercial suppliers in the US.
The PacSeal 601 is approximately $15 from McMaster.com. A Viton version PS601V is available from Superior Seal for about $35 in quantities.
An equivalent seal for this modification is available in Australia from Pump Seal Supply,(02) 9773 3084, Unit 7-9 Ladbroke Street, Milperra NSW. The seal is a .750 Shaft Size, Head Type 68, and .406 seal thickness. The price is $10.00.
Modified Timing Cover
Note that this means that the modification can only be performed with the engine out of the car.
The Final Result
I have posted drawings so you can do this modification yourself. If you don't have the confidence to do it yourself, I can provide a kit. See Ordering machined parts for ordering machined covers.