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This section refers to leaks found in the Ford power-assist style power steering system. With a few exceptions, it does not pertain to cars with integral-style power steering or rack & pinion systems.
The most common complaint about the Ford power-assist system is fluid leakage.  It is not uncommon for a system to leak due to worn and aged seals or physical damage, but often leaks continue even after parts have been repaired or replaced.  Though the Ford power-assist system was not an inherently "leak prone" system, there are common areas that seem to give problems.  In almost all cases, these leaks are caused by parts wear, physical damage, improper repair or defective parts.  This section goes over some of the more common complaints and their causes and repairs.
Control Valve/Ball Stud Assembly:
The Control Valve is the brains of the system and all pressurized fluid goes through and is directed by the valve.  Including ports, seals and o'rings, there are eight places where fluid can leak from.
1 )  LEAK:  Fluid dripping from the ball stud area is usually caused by failure of the inner spool valve seal.  This allows fluid into the ball stud area. Though the ball stud area has a rubber boot on it and is fairly sealed from moisture and dust, steering fluid is not supposed to be in this area and will find a way out. CAUSE: The rubber spool seal will get hard from age and heat and will not seal well anymore. Most often, the part of the spool valve that slides in and out of this seal becomes pitted from rust or scratched by the seal or contaminants and damages the seal, even a new one.  FIX:  Replace the spool seals, which generally means a complete control valve rebuild. If the spool valve is pitted or scratched, it must be machined down to a smooth surface, or if machining will take the spool to far undersize, replaced.
Sometimes the outer spool valve seal is the problem and the fluid will travel down the inside of the spool into the ball stud area.  The causes and repair would be the same as above.
2 )  LEAK:  Fluid leaking from under the aluminum end cap or the adapter plate between it and the control valve housing.  CAUSE:  Failure of the outer spool valve seal or the o'ring on the reaction valve plug on that end.  FIX:  Repair of the spool valve as listed above or replacement of rubber o'ring on the reaction valve plug.  Sometimes the area just inside of the reaction valve passage gets pitted from rust and the o'ring does not seal well.  Also, the o'ring is not a standard hardware store size and so replacement with the wrong size will allow the plug to leak. If the housing is badly pitted where the o'ring seals, it may need to be replaced.
3 )  LEAK:  Fluid leaking from hose connections on control valve housing, Probably the most common complaint after a rebuild.  CAUSE: Hose end flare is not sealing properly to the seat.  Many things cause this to happen - use of old and damaged seats, use of hoses with damaged or out-of-round flare surfaces, over-tightening of fittings, improper installation of new seats in housing, improper tightening procedure on fittings, stripped threads in port holes, port holes are out-of-round or cracked.  FIX:  Replace hose seats - always during a rebuild. Install properly. Do not press them in place by threading a bolt down into the port like the Ford shop manual says.  This will damage the top edge of the seat, possibly distort the seat and will shed metal particles into the valve.  Use new hoses or inspect old hose flares. The flare area must be round, level and smooth. Any burrs or defects on the flare surface will damage the seat and cause a leak. If the fitting doesn't turn on the metal tube, the fitting is distorted and the flare may be also.  Do not over-tighten hoses.  The fittings should be snug and firm but not tightened down hard. This will deeply groove the seat surface and may cause leakage.  If ports are out-of-round, cracked or have stripped threads, the flare may not seat properly.
If you have new seats and hoses and believe that they have been tightened properly but still leak, try loosening the fitting and slightly rotating the metal tube before re-tightening. This will set the flare on the seat a little differently where it might have a better seal.
NEVER use a pipe-thread fitting or adapter in a control valve housing hose port. This type of fitting seals because they are an expanding thread, not because they mate to a seat surface. Use of a pipe-thread fitting will cause the threaded hole to expand and crack.  Do not use sealer on hose fittings.  The seal is accomplished by the pressure of the flare surface of the metal hose to the seat inside the valve port - not by the threads of the fitting. 
Occasionally the pressure and return hoses will have a problem where the fittings keep loosening and will not stay tight.  This is caused by the hoses moving around during turning and flexing the metal tubing around the fittings and causing them to loosen. The 1965/1966 Mustangs had a metal strap that went around the centerlink and metal sections of the hose near the ball stud and was crimped in place. The 1967/1970 Mustangs usually had a simple screw-type hose clamp. Go
HERE for pictures. This strap was tightened down and kept the metal portion of the pressure and return hoses securely fastened against the centerlink. This kept the metal areas from moving and loosening the fittings. The same thing can be done using a heavy duty "tie wrap".  Position around the metal lines and centerlink and pull tight.
Power Cylinder:
1 )  LEAK:  Fluid leaking from stretch boot and from seal where rod goes into cylinder. CAUSE:  The seals where the rod goes into the cylinder are not sealing.  There are two seals inside the cylinder snout that seal to the chrome rod that goes inside.  They may leak due to age and heat which can cause them to get hard and shrink. Most commonly, they leak because they are damaged by the rod or cannot seal to a damaged rod.  The rod will often get pitted from rust and corrosion. The rod surface can become scratched from contact with the cylinder housing opening. The rod surface can also have an area of its hard-chome surface worn away from the same contact. The rod can become bent from a collision or running over something on the road.  Any of these problems will cause the seals to fail, even new ones.  FIX:  Replace the seals and related parts.  If the rod surface (where it goes through the seals) is smooth and free of defects, a re-seal will usually solve the problem.  If the rod is damaged in any of the ways described above, the rod will have to be replaced before new seals will do any good.  The cylinder has to be cut apart on a lathe and re-welded to do this. Always install a stretch boot on a power cylinder. The boot protects the rod from dust, road debris and water.  Without a stretch boot, the rod will become nicked and dirty which will rapidly destroy the seals.
2 )  LEAK:  Fluid leaking where the hoses go into the cylinder.  CAUSE & FIX:  Same reasons as listed above under CONTROL VALVE  #3.
3 )  LEAK:  Rarely, a power cylinder will leak in the weld where either end is welded to the main tube.  CAUSE:  This is caused by a poor or thin weld and results in a pinhole leak.  FIX:  Sometimes this leak can be repaired by applying J-B Weld to the area, but it is best fixed by having a welder put a small "tack weld" onto the hole.
Power Steering Pump:
From the late 1950's to 1977, Ford used either the Eaton-style or the Ford/Thompson-style pumps.  Each had unique areas where a leak could develope as well as areas in common.
1 )  LEAK:  Fluid leaking from the front seal at the input shaft.  CAUSE:  The seal can become hard and shrink from years of wear and heat. In this case, the seal should be replaced which can be done without disassembling the rest of the pump (if you have the correct tools).  Nowadays, this situation is agravated by a worn and grooved input shaft.  If the front seal gets hard or gets dirt trapped under the lip, this causes the surface of the shaft to wear and will form a groove in the hard-chrome surface of the shaft.  Sometimes the shaft surface will get pitted from rust and corrosion as a result of moisture clinging to the seal.  Many times the grooving or corrosion is not too bad and the shaft can be polished to an adequately smooth finish. Sometimes a replacement seal can be installed in the front housing in such a way that it contacts the shaft in an undamaged area.  FIX:  Unfortunately, most pumps have been rebuilt many times and have considerable shaft damage.  If the area of the input shaft where the seal rides is too badly or widely damaged, the shaft must be replaced.
2 )  LEAK:  A Ford-style pump that leaks around the pressure fitting or mounting stud on the back of the pump.  CAUSE:  Around the pressure fitting, there is a paper/fiber or rubber-coated metal washer that sits between the inner pressure housing and the outer canister.  These washers seal the pump when the outer nut is tightened down on the pressure fitting.  Some mid-sixties pumps also had a threaded stud that came out of the back of the pump that connected to a bracket for support on the back of the pump.  This stud had a copper washer installed between the inner housing and the outer canister.  It was sealed by tightening down a unique flat hex nut against the canister.  FIX:  Often a leak from either of these areas can be fixed by tightening down the appropriate nut.  If this doesn't solve the problem, the sealing washer on the inside is damaged or the hole in the canister is damaged. To repair either problem requires that the outer canister be removed from the front housing.  If the washer is simply torn or compressed, installing a new washer and careful reasembly will fix the problem.  Sometimes the hole in the outer canister is no longer flat and this area must be flattened out so the washer and nut will seal the canister.  Sometimes the hole becomes elongated and out-of-round.  Usually this can still be repaired if the area is carefully flattened out and a new washer installed.  If the hole is badly damaged or the edges are cracked, the canister must be replaced.
The main reason the pressure fitting hole in the canister becomes damaged is because the outer nut is tightened down without anything underneath.  The factory usually had an ID tag located here and the nut was tightened down on it. The ID tag acted like a washer under the nut. Without this tag or a washer, the nut tends to distort and tear the thin metal of the canister housing. Always use an ID tag or washer under the pressure fitting nut to protect the casnister and to spread the force of the nut over the hole in the canister.
3 )  LEAK:  A Ford-style pump that leaks fluid around the outer edge of the canister where it slips over the front housing.  CAUSE:  There is a large rubber o'ring that fits in a groove on the outer edge of the front housing and the canister slides over it.  If this seal starts leaking after years of use, it has probably become shrunken and hard from the heat.  If it leaks after installing a new seal, the canister may not have been installed properly and the o'ring may have been pinched or nicked.  Sometimes the inner lip of thecanister gets pitted from moisture right where the o'rings seals to it.  FIX:  In most instances the outer canister must be removed and the o'ring replaced.  If it is nicked or pinched during installation, check the canister flange for burrs or sharp areas. Lubricate the o'ring and the inner surface of the cannister with Lubriplate to help the can slide over the seal onto the front housing.  Check the inner surface of the cannister for roughness or rust pitting.  Smooth these out with sandpaper.  Large pits can be filled with J-B Weld and sanded down.  The inner surface of the cannister must be smooth for the o'ring to seal to it.  No not use silicone sealer to seal the o'ring.  This is a sloppy and amateurish fix and will only be temporary at best.
4 )  LEAK:  Leakage where the hoses connect to the pump.  CAUSE:  This is almost always caused by a loose hose fitting.  Sometimes if the fitting is overtightened the hose will continue to leak even after tighteneing.  Occasionally the flare of the pressure fitting will be damaged and not seat properly, even to a new hose.  Hoses run close to the exhaust manifolds and can burn through and spray fluid around in such a manner that it is hard to tell just where the leak may be.  FIX:  Make sure hose fittings are sufficiently tight on the pump.  Make sure the slip-on return hose is far enough onto the pump nipple that the hose clamp can properly afix it.  Make sure the flare surface of the pressure fitting and the hose end is smooth, burr free and undistorted.  Make sure the hoses are clear of the exhaust manifolds during the full movement of the steering linkage. 
Many early Ford pumps (such as on the 1965-1966 Mustang) had a return hose nipple that angles downwards toward the lower frame rail. Many replacement pumps for these cars do not have the correct canister on them and have a return nipple that points unpwards toward the fender edge.  This will often cause the return hose to be bent and kinked where it goes onto the return nipple.  When the hose gets old and brittle, this is where it will crack and leak first.  This upward turned return hose nipple also causes the hose to be too short for full movement of the steering linkage.  On a full left-hand turn the hose will often be stretched to the limit of its travel.  Not only can this cause the hose to crack or break at the bend near the pump, it can also put a strain on the return nipple itself and cause it to bend, crack or tear off the pump.  Always check this area of the return hose and the return nipple for cracks or damage.