Measure lining wear here Housing Disk #40 twist drill (0.098 inches in  diameter) used as visual reference Lining 13-68 Figure 13-115.  A #40 twist drill laid next to the brake lining  indicates when the lining needs to be changed on a Cleveland brake.  Figure 13-113.  Brake lining wear on a Goodyear brake is  ascertained by measuring the wear pin of the automatic adjuster. Figure 13-114.  The distance between the brake disc and the  brake housing measured with the brakes applied is a means for  determining brake lining wear on some brakes. Bleeding Master Cylinder Brake Systems Air in the Brake System Many brake assemblies contain a built-in wear indicator  pin. Typically  the exposed pin length decreases as the  linings wear  and a minimum length is used to indicate the  linings must be replaced. Caution must be used as different  assemblies may vary in how the pin measured. On the  Goodyear brake described above  the wear pin is measured  where it protrudes through the nut of the automatic adjuster  on the back side of the piston cylinder.  [Figure 13-113] The Boeing brake illustrated in  Figure 13-88  measures the  length of the pin from the back of the pressure plate when  the brakes are applied (dimension L). The manufacturer’s  maintenance information must be consulted to ensure brake  wear pin indicators on different aircraft are read correctly.  On many other brake assemblies  lining wear is not measured  via a wear pin. The distance between the disc and a portion of  the brake housing when the brakes are applied is sometimes  used. As the linings wear  this distance increases. The  manufacturer specified at what distance the linings should  be changed.  [Figure 13-114]  On Cleveland brakes  lining wear can be measured directly   since part of the lining is usually exposed. The diameter  of a number 40 twist drill is approximately equal to the  minimum lining thickness allowed.  [Figure 13-115] Multiple disc brakes typically are checked for lining wear  by applying the brakes and measuring the distance between  the back of the pressure plate and the brake housing.  [Figure 13-116]  Regardless of the method particular to each  brake  regular monitoring and measurement of brake wear  ensures linings are replaced as they become unserviceable.  Linings worn beyond limits usually require the brake  assembly to be removed for replacement. Brake systems with master cylinders may be bled by gravity  or pressure bleeding methods. Follow the instructions  in the aircraft maintenance manual. To pressure bleed a  brake system from the bottom up  a pressure pot is used.  [Figure 13-117]  This is a portable tank that contains a supply  The presence of air in the brake system fluid causes the brake  pedal to feel spongy. The air can be removed by bleeding to  restore firm brake pedal feel. Brake systems must be bled  according to manufacturers’ instructions. The method used  is matched to the type of brake system. Brakes are bled by  one of two methods: top down  gravity bleeding or bottom  up pressure bleeding. Brakes are bled when the pedals feel  spongy or whenever the brake system has been opened.