the invention is now described in detail , as follows . fig3 is a section view of the apparatus in accordance with one embodiment of the present invention , showing the pressure regulating device 200 . fig4 shows a pressure regulating device 200 that is directly connected to the hole 150 of the master cylinder 100 by threadable connection . fig5 shows a pressure regulating device 200 ′ that is connected to holes 150 of the master cylinder 100 via connecters 300 and a pipes 310 . in fig3 and fig4 the pressure regulating device 200 comprises a piston 220 with a head 222 inserted in the upper cylinder 211 of the housing 210 , such that the piston 220 and head 22 are capable of moving up and down within the upper cylinder 211 , as viewed in fig3 . a pressure chamber 228 is formed between the upper surface 212 of the housing 210 and the head 222 of the piston 220 . a sealing member 224 is placed in a groove 223 formed in the intermediate of head 222 of the piston 220 in order to prevent hydraulic fluid leakage . a fixing member 250 is threadably attached to the lower part of the housing 210 such that the lower end of the rod 226 of the piston 220 is inserted in the concave portion of the projecting extension 252 formed in the fixing member 250 . one or more openings 253 in the wall of the projecting extension 252 allow air flow into and out of the concave portion of the projecting extension 252 . a spring 230 is compressibly placed between an expansion portion 225 of the piston 220 and the fixing member 250 so as to support the piston 220 , with the lower end of the rod 226 of the piston 220 located at set distance from the bottom of the concave portion of the projecting extension 252 . if the pressure of pressure chamber 228 is higher than the pre - pressure of the spring 230 , the piston 220 drops to the bottom of concave portion of the projecting extension 252 . the expansion portion 225 of the piston 220 is stopped by the shoulder 213 of the housing 210 , which limits the rising motion of the piston 220 , as viewed in fig3 . the pre - pressure and the spring constant vary , depending on the vehicle model and a pressure controller pad 240 having an “ o ” like shape , which is arranged between the spring 230 and the upper surface of the fixing member 250 . the pressure controller pad 240 controls any unbalance of the spring 230 where the spring 230 abuts the fixing member 250 , which is threadably connected to the housing 210 . in addition , the pressure controller pad 240 reduces the elastic resistance of spring 230 . the pressure chamber 228 in the housing 210 has an inlet hole 270 for allowing hydraulic fluid to flow to the tandem master cylinder , and an air discharge valve 275 for discharging air from the pressure chamber 228 . as shown in fig4 in the embodiment in which the apparatus is directly connected to the brake master cylinder , an opening 270 is formed in the horizontal surface 212 of the cylinder 211 , as shown in fig3 and the air discharge valve 275 is located on a side wall of the upper cylinder 211 . the embodiment of the pressure regulating device 200 ′ indirectly connected to the master cylinder 100 , as shown in fig5 is connected to the master cylinder 100 by a connecter 300 and hydraulic lines 310 . the pressure regulating device 200 ′ includes two housing portions 210 ′ and 210 ″ and a connecting body 315 , which are formed into a single unit . the device 200 ′ of this embodiment is similar to the pressure regulating device 200 , as described in fig3 and 4 , except that the two housing portions 210 ′ and 210 ″ are part of a single unit , and the positions of the inlet hole 270 and the air discharge valve 275 are reversed preferably , in this embodiment , the pressure regulating device 200 ′ is connected to the tandem brake master cylinder 100 by connecting portions that are connected at similar points in the hydraulic flow relative to the primary and the secondary ring figured cups as the embodiment in which the device 200 is directly connected to the brake master cylinder . the operation of the pressure regulating device 200 ′ in application for a tandem brake master cylinder on a motor vehicle will now be described . as shown in fig3 and 5 , after the pressure regulating device 200 or 200 ′ is connected to the tandem brake master cylinder 100 , the brake pedal 1 is depressed and released repeatedly to bleed air from the system . through this process , when the brake pedal 1 is depressed , brake fluid in the tandem brake master cylinder 100 flows from one opening to the wheel cylinder 7 ′ and from another opening to the pressure chamber 228 . as this occurs , air in the pressure chamber 228 is discharged through the air discharge valve 275 . as the pedal 1 is depressed , when the lining 9 ′ of the brake shoe contacts the brake drum 13 ′, the pressure in the tandem brake master cylinder 100 increases . at this point , if the pressure in the tandem brake master cylinder 100 is higher than the desired pressure as governed by the device 200 or 200 ′, the pressure in the pressure chamber 228 increases until this pressure exceeds the resistive pressure provided by the spring 230 , which includes a loaded pre - pressure . upon this pressure in the chamber 228 exceeding the resistive pressure of the spring 230 , the spring 230 compresses , and the piston 220 moves to the bottom of the concave portion of the projecting extension 252 . upon the pedal 1 being released , by function of the return spring in the tandem brake master cylinder 100 , the primary ring figured cup 101 and the secondary ring figured cup 102 move toward the brake pedal 1 , and the inner pressure within tandem brake master cylinder 100 falls abruptly . at this moment , the restoring force of the spring 230 within the pressure regulating device 200 or 200 ′ exceeds the pressure of pressure chamber 228 , and the brake fluid stored in the pressure chamber 228 flows into the tandem brake master cylinder 100 up to the limit provided by volume of the space 257 . as a result of this operation , brake fluid from the pressure regulating device 200 or 200 ′ returns to the tandem brake master cylinder 100 before fluid from the wheel cylinder 7 ′ returns to tandem brake master cylinder 100 because the pressure in pressure chamber 228 resulting from the pressure of the spring 230 is higher than the pressure typically present within the tandem brake master cylinder 100 . this is further the case because the pressure within the brake master cylinder 100 will have increased , such that this pressure exceeds a desired pressure , as regulated by a check valve 330 , resulting in the check valve 330 closing . because the check valve 330 so closes , with some amount of brake fluid remaining within the wheel cylinder 7 ′, the gap between the brake drum 13 ′ and the brake lining 9 ′ is maintained at a minimum . as indicated above , the bleeding of air from the brakes also helps to maintain a minimum distance between the drum 13 ′ and the lining 9 ′, while keeping the brake fluid in the pressure chamber 228 of the pressure regulating device 200 or 200 ′ full . after air is bled from the brakes , if the pedal 1 is slowly depressed , the gap between the drum 13 ′ and lining 9 ′ remains narrow without the pressure regulating device 200 or 200 ′ operating , because the pressure in the tandem brake master cylinder 100 under this condition does not exceed the pressure of the spring 230 , which is set with a loaded pre - pressure that is greater than the pressure of the pressure regulating device 200 or 200 ′. in an emergency , when the brake pedal 1 is urgently depressed , pressure in the tandem brake master cylinder 100 exceeds that of the spring 230 , including the loaded pre - pressure , and brake fluid in the tandem brake master cylinder 100 flows into the pressure chamber 228 of the pressure regulating device 200 or 200 ′. because , in conditions such as the emergency conditions described above , brake fluid flows into the pressure chamber 228 after the lining 9 ′ contacts the brake drum 13 ′, the pressure in the tandem brake master cylinder 100 prevents abrupt pedal stroke motion a of the brake pedal 1 , as shown in fig4 . in addition , because the piston 220 is able to move as much as the space 257 of the pressure regulating device 200 or 200 ′ allows , the brake lining 9 ′ comes into contact more slowly with the brake drum 13 ′ than would occur absent the present invention , and , as a result , the revolution of wheels are stopped but skid is reduced . the extent to which this effect occurs varies as a function of the power applied to the pedal 1 in an emergency , the speed of the vehicle , the cross - sectional area of the tandem brake master cylinder 100 , the cross - sectional area of the piston 220 in the pressure regulating device 200 or 200 ′ relative to the brake pedal &# 39 ; s stroke a , the size of the space 257 in the concave portion of the projecting extension 252 , and the pre - pressure applied by and the spring constant of the spring 230 . as discussed above , if a vehicle is stopped without wheel lock , steering is improved relative to locked wheel conditions , braking safety is enhanced , the rate of vehicle stopping is increased , and stopping distance is reduced . as described above , the present invention provides an apparatus for regulating pressure in the hydraulic brake system such that stopping distance is decreased by maintaining a narrowed gap between the brake drum and the brake lining , while braking safety for a vehicle is further enhanced by preventing wheel lock when the brake pedal is abruptly applied . it will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof . therefore it is understood that this invention is not limited to the particular embodiments disclosed , but it is intended to cover modification within the spirit and scope of the present invention as defined by the appended claims .