Division 2.4 Symbols valid for hydraulic-transmission braking systems (see Figure 8 of Appendix 1)

reduction ratio between travel of coupling head and travel of piston in master cylinder;

reduction ratio between travel of cylinder thrust point and lift (application travel) of brake-shoe centre;

surface area of piston of one wheel cylinder for drum brake(s); for disc brake(s), sum of the surface area of the caliper piston(s) on one side of the disc;

surface area of piston in master cylinder;

hydraulic pressure in brake cylinder;

retraction pressure in the brake cylinder when the towed vehicle moves forward; i.e., in graph of M = f(p), the value of the pressure p at the point of intersection of the extrapolation of this function with the abscissa (see Figure 7 of Appendix 1);

brake retraction pressure when the towed vehicle moves rearward (see Figure 7 of Appendix 1 );

hydraulic pressure in the brake cylinder to produce the braking force B*;

test pressure according to point 6.2.1:

characteristic of the brake when the towed vehicle moves forward as defined from:

M = ρ′ (p – p_o)

characteristic of the brake when the towed vehicle moves rearward as defined from:

M_r = ρ′_r (p_r – p_or)