certain terminology is used herein for convenience only and is not to be taken as a limitation on the invention . for example , words such as “ upper ,” “ lower ,” “ left ,” “ right ,” “ horizontal ,” “ vertical ,” “ upward ,” and “ downward ” merely describe the configuration shown in the figs . indeed , the components may be oriented in any direction and the terminology , therefore , should be understood as encompassing such variations unless specified otherwise . referring now to the drawings , wherein like reference numerals designate corresponding or similar elements throughout the several views , an embodiment of a fluid dispensing apparatus according to the present invention is shown in fig1 and generally designated at 10 . the apparatus 10 comprises a head 12 , a container 14 , a pistol - grip 16 including a pivoting lever 18 and a handle 20 rigidly attached to the head 12 , and a reciprocating piston 22 connected at one end to the lever 18 and extending into the head 12 . a spring 24 surrounding the piston 22 is positioned between the head 12 and the lever 18 for biasing the lever 18 and piston 22 away from the head 12 . a push - button air pressure release valve 26 of the type typically used in the art extends through the head 12 for the purpose of relieving air pressure within the device 10 . when a user desires to dispense fluid , the user holds the pistol grip 16 with the palm of the hand on the lever 18 and fingers around the handle 20 . applying pressure moves the lever 18 toward the handle 20 and drives the piston 22 into the head 12 for pumping fluid from an outlet conduit 28 , as will be described below . the structure and operation of the pistol - grip 16 is described in u . s . pat . no . 5 , 277 , 339 , the contents of which are hereby incorporated by reference . accordingly , a detailed explanation of the pistol - grip 16 is deemed unnecessary for understanding of the present invention by one of ordinary skill in the art . however , it is understood that any pistol - grip or lever - type actuating mechanism may be used to reciprocate the piston 22 for dispensing lubricant in accordance with the present invention . moreover , although a rigid pipe is depicted as the outlet conduit 28 in the figs ., a flexible outlet hose or any other suitable conduit may be used . as shown in fig2 a piston chamber 30 is formed inside the head 12 . the piston chamber 30 is open at one end of the head 12 for receiving the piston 22 . a first portion 32 of the piston chamber 30 proximal the open end is wider in diameter than a second distal portion 34 of the piston chamber 30 . the piston 22 ( fig3 ) has a wider intermediate portion 36 which fits within the first portion 32 of the piston chamber 30 . as seen in fig2 the piston 22 extends partially into the second portion 34 of the piston chamber 30 when the fluid dispensing apparatus 10 is not in use . an o - ring 38 is attached in a peripheral groove 40 on the wider intermediate portion 36 of the piston 22 to sealingly engage the walls of the first portion 32 of the piston chamber 30 . similarly , an o - ring 42 is attached in a peripheral groove 44 adjacent the distal end 46 of the piston 22 to sealingly engage the walls of the second portion 34 of the piston chamber 30 . the o - rings 38 , 42 attached to the piston 22 provide fluid - tight separation of the piston chamber 30 while allowing the piston 22 to reciprocate linearly relative to the walls of the piston chamber 30 due to material deformation of the o - rings 38 , 42 . the head 12 , pistol grip 16 and piston 22 are preferably made of metal or of a synthetic polymeric material . suitable synthetic polymeric materials are those that are rigid , including but not limited to , polycarbonate , polyethylene terephthalate ( pet ), high density polyethylene ( hdpe ) and the like . nevertheless , any material of sufficient strength to withstand the forces the fluid dispensing apparatus 10 may encounter in use may be used . moreover , the use of the o - rings 38 , 42 attached to the piston 22 eliminate the need for a precision cut piston chamber 30 and , thus , allows for the head 12 to be inexpensively manufactured . it is understood that the scope of the invention is not intended to be limited by the materials listed herein , but may be carried out using any materials that allow the construction and operation of the described fluid dispensing apparatus 10 . an air inlet duct 48 extends through the front wall of the head 12 and opens into the first portion 32 of the piston chamber 30 . an elastomeric ball check valve 50 positioned in the air inlet duct 48 permits air to flow only from the ambient into the piston chamber 30 . an air outlet duct 52 extends from the first portion 32 of the piston chamber 30 and through the rear wall of the head 12 . an elastomeric ball check valve 54 positioned in the air outlet duct 52 permits air to flow only out of the first portion 32 of the piston chamber 30 . a fluid inlet duct 56 extends through the rear wall of the head 12 and opens into the second portion 34 of the piston chamber 30 . a metal ball check valve 58 positioned in the fluid inlet duct 56 permits fluid to flow only into the second portion 34 of the piston chamber 30 . a fluid outlet duct 60 extends from the second portion 34 of the piston chamber 30 and through the front wall of the head 12 . a metal ball check valve 62 positioned in the fluid outlet duct 60 permits fluid to flow only out of the second portion 34 of the piston chamber 30 . the fluid outlet duct 62 is adapted to threadably receive the outlet conduit 28 . although ball check valves are shown , it is understood that other suitable check valves , such as flap check valves and the like , may be used . the check valves can be manufactured from metal , plastic , elastomeric material , or any other suitable material . preferably , the valves 50 , 54 positioned in the air inlet and outlet ducts 48 , 52 are formed from an elastomeric material . because petroleum based fluids may not interact well with elastomeric material , the valves 58 , 62 positioned in the lubricant inlet and outlet ducts 56 , 60 are preferably composed of metal . alternatively , the air and fluid outlet ducts 52 , 60 can be longitudinally spaced toward the closed end of the piston chamber 30 relative to the air and fluid inlet ducts 48 , 56 , respectively . thus , as the piston 22 moves into the piston chamber 30 , the periphery of the piston 22 will seal the air and fluid inlet ducts 48 , 56 . in this arrangement , check valves need not be provided in the air and fluid inlet ducts 48 , 56 . the rear of the head 12 comprises a circular threaded flange 64 defining a recess 66 . the threaded flange 64 is preferably composed of the same material used for the head 12 . however , it is contemplated that the threaded flange 64 could be composed of a material different from that of the head 12 as long as the chosen material permits the container 14 to sealingly engage the circular threaded flange 64 of the head 12 , as will be described below . referring to fig4 the container 14 has an open first end 68 and a closed second end 70 and is adapted for containing fluid 72 to be dispensed . an elongated hollow air delivery tube 74 and a cup - shaped follower 76 are disposed inside the container 14 . an o - ring 78 is seated in a peripheral groove 80 on the outside of the follower 76 to ensure that the follower 76 sealingly engages the inner wall of the container 14 . at the center of the follower 76 is an opening 82 designed to permit the air delivery tube 74 to slidingly pass through the follower 76 . the container 14 , hollow air delivery tube 74 , and follower 76 are preferably made of a metal or synthetic polymeric material . suitable synthetic polymeric materials are those that are rigid , including but not limited to , polycarbonate , polyethylene terephthalate ( pet ), high density polyethylene ( hdpe ) and the like . nevertheless , any material of sufficient strength to withstand the forces the fluid dispensing apparatus 10 may encounter in use may be used . the open first end 68 of the container 14 is externally threaded for coupling with the circular threaded flange 64 on the head 12 . as best seen in fig2 an o - ring 84 is seated in a groove 86 in the flange 64 for sealing the connection between the head 12 and the container 14 . the air delivery tube 74 connects at one end to the air outlet duct 52 in the head 12 and extends longitudinally from the open first end 68 of the container 14 to the closed second end 70 of the container 14 . an o - ring 88 is seated in a groove 90 inside the air outlet duct 52 to sealingly engage the air delivery tube 74 . an o - ring 92 is contained within the opening 78 of the follower 76 to ensure that the area between the follower 76 and the air delivery tube 74 is sealed . the thickness of the peripheral wall 94 of the follower 76 tapers rearwardly to promote low friction movement of the follower 76 relative to the container 14 and to allow some deflection , as necessary , to counteract any irregularities in the shape of the container 14 . thus , the volume defined by the closed end 70 of the container 14 and the follower 76 is sealingly separated from the volume on the other side of the follower 76 at the open end 68 of the container 14 . the fluid 72 to be dispensed is placed in the container 14 between the follower 76 and the open first end 68 of the container 14 . it is anticipated that a wide variety of fluids may be dispensed using the apparatus 10 of the present invention , including , but not limited to , for example , lubricants such as grease and other high viscosity fluids or semi - solid materials that require high pumping pressure to achieve adequate flow rates such as caulk , glue , cake frosting and the like . in use , the lever 18 attached to the head 12 is manually actuated toward the handle 20 , moving the piston 22 into the piston chamber 30 in the head 12 as illustrated in fig5 . the movement of the wider intermediate portion 36 of the piston 22 in this direction forces air from the first portion 32 of the piston chamber 30 through the air outlet duct 52 and air delivery tube 74 to the space between the follower 76 and the closed end 70 of the container 14 . the increase in air pressure forces the follower 76 to the left ( as seen in fig5 ) in the container 14 and thereby forces the fluid 72 towards the open first end 68 of the container 14 . similarly , the movement of the distal end 46 of the piston 22 farther into the second portion 34 of the piston chamber 30 forces fluid in the second portion 34 of the piston chamber 30 through the fluid outlet duct 60 where it is emitted from the outlet conduit 28 . as illustrated in fig6 when pressure on the lever 18 is released , the spring 24 surrounding the piston 22 forces the lever 18 away from the handle 20 , moving the piston 22 out of the piston chamber 30 to its original position ( as seen in fig2 ). a vacuum is created in the piston chamber 20 by the movement of the piston 22 , drawing fluid through the fluid inlet duct 56 into the second portion 34 of the piston chamber 30 and air from the ambient through the air inlet duct 48 into the first portion 32 of the piston chamber 30 . as described above , the o - ring 38 on the wider intermediate portion 36 of the piston 22 prevents air entering the first portion 32 of the piston chamber 30 from passing out of the open end of the piston chamber 30 and the o - ring seal 42 on the distal end 46 of the piston 22 prevents air from entering into the second portion 34 of the piston chamber 30 . when the fluid 72 in the container 14 has been fully dispensed , or when the user desires to dispense a different fluid , the user must reload the apparatus 10 . to reload , the user removes the container 14 from the threaded flange 64 on the head 12 and detaches the air delivery tube 74 in the container 14 from the air outlet duct 52 . the user then attaches a different container 14 and its components to the head 12 by inserting the air delivery tube 74 into the air outlet duct 52 and fastening the new container 14 to the head 12 . in an alternative embodiment of the apparatus 10 , the air delivery tube 74 is permanently attached to the air outlet duct 52 . in this embodiment , when the user removes the container 14 from the head 12 , the air delivery tube 74 slides out of the follower 76 . when the user then attaches a different container 14 to the head 12 , the user must insert the air delivery tube 74 through the opening 82 in the follower before fastening the container 14 to the head 12 . it is contemplated that when an inexpensive material is used for the container 14 and its components , the user , upon dispensing all of the fluid 72 contained therein , may dispose of the entire container 14 and its contents when spent . this serves to make the process of reloading the apparatus 10 significantly easier and cleaner . moreover , the use of an easily removable container 14 also allows for different sizes of containers 14 to be utilized with the device 10 . thus , so long as the air pressure generated by the piston is sufficient to pressurize a predetermined volume of fluid in the container 14 , the user can utilize different sized containers 14 adapted to attach to the head 12 containing different volumes of fluid 72 depending on the extent of the job required . further , at least a portion of the container 14 may be translucent so that the user can see through the container 14 to determine the type and amount of fluid 72 contained therein . in an embodiment of the fluid dispensing apparatus where the head 12 , pistol grip 16 , and reciprocating piston 22 , are also made of an inexpensive material , the entire apparatus 10 can be manufactured for low cost . the use of an inexpensive material for the all of the components would , thus , allow for the user , upon dispensing all of the fluid 72 contained therein , to dispose of the entire apparatus 10 without having to ever be exposed to the fluid 72 in the container . although the present invention has been shown and described in considerable detail with respect to only a few exemplary embodiments thereof , it should be understood by those skilled in the art that we do not intend to limit the invention to the embodiments since various modifications , omissions and additions may be made to the disclosed embodiments without materially departing from the novel teachings and advantages of the invention , particularly in light of the foregoing teachings . for example , the air delivery tube for transporting air from the head to the space between the follower and the closed second end of the container could be external from the container . accordingly , we intend to cover all such modifications , omission , additions and equivalents as may be included within the spirit and scope of the invention as defined by the following claims . in the claims , means - plus - function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures . thus , although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together , whereas a screw employs a helical surface , in the environment of fastening wooden parts , a nail and a crew may be equivalent structures .