This invention relates to an air brake system, particularly but not exclusively for railway trains, of the well-known direct releasing kind in which a controlled fall ^■ in pressure in a brake pipe determines the auxiliary reservoir pressure admitted to a brake cylinder, thereby achieving a predetermined braking effect, and a subsequent increase in the brake pipe pressure causes an abrupt release of the brakes.

A known direct releasing air brake system of the kind defined has a distributor with a main control valve part comprising a main chamber divided into two sub-chambers by a diaphragm assembly at the lower end of a valve stem having a hollow upper end section which in the normal running condition with the brakes released permits venting of the brake cylinder pressure to atmosphere through the intermediate chamber. The upper end of the stem section is arranged to lift the main control valve element off its seat when the diaphragm is deflected upwardly, thereby enabling the auxiliary reservoir pressure to be supplied to the brake cylinder pipe, the hollow stem section being sealed by contact with the valve element. Upward deflection of the diaphragm is caused by a driver- controlled drop in brake pipe pressure which is effective in the upper sub-chamber until the fall in auxiliary reservoir pressure effective in the lower sub-chamber causes a partial return of the diaphragm into what is known as the lap condition of the distributor in which the main control valve element is seated but remains in sealing contact with the hollow valve stem section in order to prevent venting of the brake cylinder pressure to atmosphere.

It is known to provide in such an air brake distributor a sealing valve for controlling a pressure equalising connection between the auxiliary reservoir and the brake pipe. When the brakes are released the sealing valve is open so allowing the auxiliary reservoir to be charged up to the same pressure as the brake pipe so that in the normal running condition with the brakes released both the brake pipe and the

auxiliary reservoir are at the same pressure. When the brakes are then applied and the main control valve operates in the way described above, a direct release valve is opened by the rising diaphragm assembly to allow brake pipe air into the sealing valve diaphragm chamber so closing the valve to isolate the brake pipe from the auxiliary reservoir. In the subsequent lap condition the return deflection of the diaphragm assembly in the main chamber is insufficient to close the direct release valve and the sealing valve therefore remains closed. On release of the brakes, as described above, the pressure increase in the brake pipe deflects the diaphragm assembly in the main chamber downwardly sufficiently to permit spring return of the direct release valve into its closed position thereby allowing immediate venting to atmosphere of the sealing valve diaphragm chamber. The sealing valve thus opens to re-establish the connection between the brake pipe and the auxiliary reservoir. In a long train, for example, with many wagons each having its own auxiliary reservoir but connected to a common brake pipe, the rise in brake pipe pressure towards the rear end of the train may be very slow with the result that rapid venting and abrupt opening of the sealing valve may re-establish the connection between the brake pipe and the auxiliary reservoir before the brakes are conditioned for release. Because the pressures in the sub- chambers of the main chamber are then balanced, further downward deflection of the diaphragm assemblies in the main chamber is prevented and the brakes may remain partially applied in some of the rear wagons of the train.

It is also known in distributors of conventional air brake systems of the direct releasing kind to provide for accelerated release of the brakes once brake release has been initiated. In our known design of direct releasing distributor, for example, accelerated release is achieved by downward deflection of the main control valve diaphragm assembly (normally resulting from a driver-controlled increase in brake pipe pressure) unseating an accelerated release inlet valve so as to connect an accelerated release reservoir to the

brake pipe thereby further raising the pressure in the brake pipe and accelerating the descent of the diaphragm assembly. A problem is that downward deflection of the main control valve diaphragm assembly may also result from a drop in the auxiliary reservoir pressure applied to the lower sub-chamber pf the main valve chamber as a result for example of leakage in the auxiliary reservoir of the particular wagon. It is known to provide a narrow maintaining port bridging the diaphragm assembly so that small reductions in the pressure of the lower sub-chamber can be directly compensated by brake pipe pressure transmitted across the diaphragm. However, the maintaining port is incapable of compensating substantial pressure drops of the auxiliary reservoir and some downward deflection of the main control valve diaphragm is therefore inevitable in these circumstances but if the accelerated release inlet valve is opened as a result of such faulty operation the resultant increase in the brake pipe pressure may be detected in the distributors of adjacent wagons so triggering a chain reaction of brake release on all wagons when in fact the brake controller or driver wishes to maintain a braking condition. In our own known distributor design, operation of the accelerated release inlet valve has therefore required a further rise in brake pipe pressure beyond that needed to initiate release but the above mentioned slow rise in brake pipe pressure in the rear wagons of a long train may inhibit accelerated release, which has led to specification of an alternative design in which the accelerated release is triggered for example by the exhaust of brake cylinder pressure through the main valve stem but this may increase the risk of an accidentally propagated brake release. Careful sizing of the accelerated release reservoir may limit the volume of air introduced into the brake pipe from this reservoir so as not to initiate release of the brakes on adjacent wagons but this cannot be relied upon and has the disadvantage that the effectiveness of the accelerated release is limited during normal operation.

It is an object of the present invention to provide an

air brake system or distributor therefor in which the aforesaid disadvantages are obviated or mitigated.

According to the present invention there is provided an air brake system of the kind defined, or a distributor therefor, wherein a sealing valve is adapted to close on .braking so as to isolate the brake pipe from an auxiliary reservoir, characterised in that on brake release opening of the sealing valve is delayed for a predetermined time interval so as to allow sufficient build-up of brake pipe pressure in the main control valve chamber to ensure full brake release preferably with accelerated release if available.

The invention will now be further described by way of example only, with reference to the following drawings, in which:-

Fig. 1 is a cross sectional diagram of an air brake distributor in accordance with the invention, shown in the braking condition;

Fig. 2 is a corresponding diagram showing the distributor in the lap condition;

Fig. 3 is a corresponding diagram showing the distributor in the brake release condition, and

Fig.4 is a corresponding diagram showing the distributor in the accelerated release condition.

The general operation of the illustrated distributor will be known to the person skilled in the art and is described in the introduction of the specification, the parts referred to in that introduction being marked in the drawings. Referring now to the drawings, as shown in Fig. 1, a driver -controlled fall in brake pipe pressure is effective in the top sub-chamber 9 of the main chamber of the main control valve to deflect the diaphragm assembly 32 upwardly and thereby unseat the main control valve element to establish a temporary direct connection between the auxiliary reservoir and the brake cylinder. At the same time the diaphragm assembly 32 lifts the direct release valve into the open position so as to apply brake pipe pressure to the upper side of the diaphragm of the sealing valve thereby seating the

sealing valve to interrupt the charging connection between the brake pipe and the auxiliary reservoir. The sealing valve has an enlarged chamber above the diaphragm as compared to existing designs but its connection to the pipe from the direct release valve is provided with a choke for the reasons .explained below.

Referring now to Fig. 2, the drop in pressure in the auxiliary reservoir as a result of its direct connection to the brake cylinder causes a partial return of the diaphragm assembly 32 into the lap condition in which the main control valve is closed but the top of the control stem remains sealed to prevent venting of the brake cylinder to atmosphere. The direct release valve remains unseated so that the sealing valve remains closed.

Fig. 3 shows the release condition in which a driver- controlled increase in brake pipe pressure transmitted to the upper sub-chamber 9 deflects the diaphragm assembly 32 downwardly from the lap condition so as to open the upper end of the control stem and permit venting of the brake cylinder to atmosphere. At the same time the direct release valve is closed under spring pressure and the pipe connecting the direct release valve to the sealing valve is immediately vented to atmosphere. In contrast to existing designs, the larger chamber volume of the sealing valve and the choke controlled opening thereof has the effect of maintaining the sealing valve in the closed condition, i.e. delaying its opening for a predetermined time interval, so preventing the unintended retention of a residual braking effect as a result of incomplete venting of the brake cylinder.

As illustrated in Fig. 4, continued descent of the diaphragm assembly 32 as a result of increased pressure in the sub-chamber 9 unseats the accelerated release inlet valve thereby connecting the accelerated release reservoir to the brake pipe pressure applied to the top of the diaphragm and accelerating the descent of the diaphragm assembly into the fully released position. Because the sealing valve remains closed the pressure differential across the diaphragm is

enhanced to an extent which would not be the case if the sealing valve were opened without delay to connect the brake pipe to the auxiliary reservoir. Thus the safety margin of excess pressure required at the top of the diaphragm and previously obtained in our design by requiring a longer build¬ up of the brake pipe pressure can be achieved more quickly and effectively by delaying the opening of the sealing valve in the manner described.