fig1 shows a braking system comprising two ( left and right ) master cylinders 10 , 10 ′ connected to two slave cylinders 12 , 12 ′ by way of respective hydraulic pressure lines 14 , 14 ′ to form separate left and right braking circuits . the two braking circuits are identical and only one of them needs to be described . in each of the braking circuits , the master cylinder 10 comprises a housing 10 a containing a piston 10 b that is moved by means of an actuating rod 10 c connected to a respective brake pedal . when the piston 10 b is moved by depressing a brake pedal , fluid under pressure is supplied through a port 10 d to the slave cylinder 12 , the piston of which acts on a brake pad on a respective side of the vehicle . each of the master cylinders 10 , 10 ′ is further connected through a port 10 e that incorporates a non - return valve 10 f , to a common reservoir 16 at ambient pressure through a line 18 . the reservoir maintains the circuits filled with hydraulic fluid as their volume increases through wear of the brake pads . as so far described , each hydraulic circuit is entirely conventional and operates in the same manner as the braking system of most road vehicles . because two separate braking circuits are provided for the left and right side of the vehicle , during field operation the left and right sides of the vehicle can be braked separately by operating only one or other of the two brake pedals and this allows the vehicle to be steered by means of its brakes . such braking is inappropriate , for obvious reasons , when the vehicle is to be driven on a road . thus , during road use , the brake pedals are mechanically connected to one another so that they cannot be operated separately . one cannot rely on the mechanical coupling of the brake pedals to ensure equal braking on both sides of the vehicle and this is instead accomplished , in a known manner , by interconnecting the hydraulic circuits . to this end , each of the master cylinders 10 and 10 ′ additionally comprises a pressure equalisation port 10 g that incorporates a non - return valve 10 h . the piston 10 b has a shoulder which acts on the closure member of the non - return valve 10 h as soon as the brake pedal is depressed to connect the working chamber of the cylinder to a pressure equalisation line 20 . if only one of the brake pedals is operated during field use , only one of the non - return valves 10 h will be open and high pressure will not be able to flow from the actuated braking circuit to the other . however if both pedals are depressed , even by unequal amounts , the two circuits will be able to communicate with one another through the pressure equalisation line 20 to ensure symmetrical braking when the vehicle is in road use . as so far described , the braking system is known in the context of brake steered agricultural vehicles . master cylinders having an additional pressure equalisation which also incorporate a non - return valve in the pressure equalisation port that is opened as soon as the piston is moved are currently available and their internal construction need not therefore be described in greater detail herein . the illustrated embodiment of the present invention comprises a solenoid operated two position valve 24 and a pressure relief valve 26 arranged in series with one another in a line 22 that leads from the pressure equalisation line 20 to the line 18 connected to the fluid reservoir 16 . the solenoid valve 24 is shown in its normal position for field use wherein it maintains the pressure equalisation line 20 isolated and does not therefore interfere with pressure supplied to the slave cylinders 12 , 12 ′. however , when a parameter is sensed that indicates that the vehicle risks toppling because it is being driven on a road with the header raised , the solenoid valve 24 is moved into its other position in which it connects the pressure equalisation line 20 to the pressure relief valve 26 . the pressure relief valve 26 is a spool valve that is acted upon in one direction by a spring and in the opposite direction by a pilot pressure derived from its intake port . as soon as the pressure applied to the relief valve 26 exceeds a threshold , which as represented by an arrow in the drawing may be adjustable to suit the vehicle , the valve opens and connects the pressure equalisation line 20 to the ambient pressure in the reservoir 16 . in this way , the pressure delivered to the slave cylinders 12 , 12 ′ is limited to the value set by the pressure relief valve and the risk of the vehicle toppling is avoided . the signal for operating the solenoid valve 24 may be derived from any suitable source , for example from a sensor that responds to the selected drive ratio , the speed of the vehicle or the height of the header . it will be seen from the drawing that the additional components required to eliminate the risk of the vehicle toppling during road use are the two contained within the box drawn in dotted lines , namely the two position solenoid valve 24 and the pressure relief valve 26 . these two components can be formed as a sub - assembly that may be connected using only two hydraulic connections to an existing braking system , thus making it possible to modify existing vehicles with relative ease . though the embodiment of fig1 is preferred because of the ease of retrofitting , the capacity of the braking system can be altered in other ways as will now be described with reference to two further embodiments of the invention , shown in fig2 and in fig3 and 4 , respectively . in the case of both these further embodiments , all the components shown in fig1 , other than the two valves 24 and 26 , are present . these are the components that are conventionally to be found in an agricultural vehicle that can be steered by asymmetrical braking . in the case of the alternative embodiment , a valve body 50 as shown in fig2 is provided , in addition to which a second brake calliper ( not shown ), which incorporates an additional slave cylinder , is provided on each wheel . lines 60 and 62 lead to the separate slave cylinders on the left side of the vehicle and lines 60 ′ and 62 ′ lead to separate slave cylinders on the right side of the vehicle . once again , primed reference numerals will be used to avoid unnecessary repetition of description . the valve body 50 shown in fig2 has two intake ports connected to the lines 14 , 14 ′ from the master cylinders 10 , 10 ′ and four output ports connected to the lines 60 , 62 , 60 ′ and 62 ′. a valve spool 52 is mounted in a bore in the valve body 50 and at each end the spool is acted upon by a spring 55 , 55 ′ and the pressure in a control chamber 54 , 54 ′ that communicates with one of the intake ports . when equal pressures are applied to the two control chambers 54 , 54 ′, the spool 52 adopts its illustrated position . here the lines 14 and 14 ′ are connected to the lines 60 , and 60 ′ through passages 66 and 66 ′ in the body 50 while the two further passages 68 and 68 ′ that lead to the lines 62 and 62 ′ are closed by the valve spool 52 . thus , when both brake pedals are depressed during road use of the vehicle , only one set of brake pads acts on each side of the vehicle . when only the left side is braked , the pressure in the control chamber 54 will move the valve spool 52 to the right as viewed , allowing fluid to flow through the passage 68 to the line 62 leading to the slave cylinder of the second calliper and two sets of brakes will be activated on the left side of the vehicle but none on the right , causing the vehicle to be steered to the left . to allow hydraulic fluid to be drained from the second slave cylinder when the brake pedal is released and the passage 68 is again closed by the valve spool 52 , a non - return valve 64 is provided which allows fluid to flow from the line 62 to the line 14 only in the direction that reduces the braking force on the wheels . by operation of the valve body 50 , it is ensured that maximum braking force can still be applied when braking only one wheel , by means of the first and second slave cylinders . the braking force will however be divided by two when braking both wheels simultaneously because only the first slave cylinder on the left and right wheel is used due to an equal pressure being applied to both sides of the valve spool 52 . the embodiment of fig3 and 4 comprises a valve 30 connected to the lines 14 , 14 ′ of fig1 and two pressure amplifiers 38 and 38 ′ each connected to a respective one of the slave cylinders 12 and 12 ′ in fig1 . the valve 30 is generally similar to the valve 50 in fig2 and has control chambers 34 and 34 ′ connected to the lines 14 and 14 ′ from the master cylinders . as in the case of the valve spool 52 in fig2 , the spool 32 of the valve 30 adopts a central position , shown in fig3 , when both brake pedals are depressed during road operation and an end position , shown in fig4 , when only one brake pedal is depressed during field operation . during road operation , the master cylinder lines 14 and 14 ′ are connected to the slave cylinders 12 and 12 ′ through lines 42 and 42 ′ that bypass the pressure amplification stages 38 and 38 ′ so that the latter have no effect . on the other hand , when the spool 32 moves to its end position shown in fig4 because only the left brake pedal is depressed , the passage 42 is blocked by a land of the spool 32 and instead the pressure from the master cylinder line 14 is applied through the passage 40 to the amplification stage 38 which increases the pressure applied to the slave cylinder 12 by a factor equal to the ratio of the surface areas of the opposite ends of its piston . similarly , on depressing the right brake pedal , the master cylinder line 14 ′ is blocked by another land of the spool 32 , preventing pressure to be transmitted to the right slave cylinder 12 ′. accordingly , when driving on the road and depressing both brake pedals simultaneously , the spool 32 is forced to stay in the middle of the valve 30 , because of the equal pressure in the master cylinder lines 14 and 14 ′. however , because of the pressure amplification stages 38 and 38 ′, the system is able to create a higher braking force when the left or right brake is used alone . the system is dimensioned such that the braking force is within safe braking limits when driving on the road .