, ▁the ▁sh unt ▁jam med ▁too ▁often , ▁causing ▁pain ▁and ▁blind ness , ▁risk ing ▁brain ▁damage ▁and ▁requiring ▁emer gency ▁surg ery . ▁Till , ▁neu ros ur geon ▁at ▁London ' s ▁Great ▁Or mond ▁Street ▁Hospital ▁for ▁children , ▁determined ▁that ▁de br is ▁accum ulated ▁in ▁the ▁hydro ce phal ic ▁vent ric les ▁could ▁c log ▁the ▁sl its ▁in ▁the ▁Hol ter ▁val ves , ▁especially ▁with ▁patients ▁such ▁as ▁The o ▁who ▁had ▁bad ▁ble eding ▁in ▁the ▁brain ▁and ▁brain ▁damage . ▁▁ ▁D ahl ▁knew ▁W ade ▁to ▁be ▁an ▁expert ▁in ▁precision ▁hyd ra ul ic ▁engineering , ▁from ▁their ▁shared ▁h ob by ▁of ▁flying ▁model ▁aircraft . ▁( In ▁addition ▁to ▁building ▁his ▁own ▁model ▁aircraft ▁engines , ▁W ade ▁ran ▁a ▁factory ▁at ▁High ▁Wy com be ▁for ▁producing ▁precision ▁hyd ra ul ic ▁p umps .) ▁With ▁D ahl ▁coordin ating ▁the ▁efforts ▁of ▁the ▁neu ros ur geon ▁and ▁the ▁hyd ra ul ic ▁engineer , ▁the ▁team ▁developed ▁a ▁new ▁mechanism ▁using ▁two ▁metal ▁disc s , ▁each ▁in ▁a ▁restrict ive ▁housing ▁at ▁the ▁end ▁of ▁a ▁short ▁sil ic one ▁rub ber ▁t ube . ▁Fl uid ▁moving ▁under ▁pressure ▁from ▁below ▁pushed ▁the ▁disc s ▁against ▁the ▁t ube ▁to ▁prevent ▁retro grade ▁flow ; ▁pressure ▁from ▁above ▁moved ▁each ▁disc ▁to ▁the ▁" open " ▁position . ▁As ▁Till ▁reported ▁in ▁The ▁Lanc et , ▁the ▁in vention ▁was ▁character ized