this sounds like you may be losing vacuum to the brake servo
now depending on the engine spec it may use a vacuum pump driven off the back of the cylinder head to drive the serov - in which ase check the condition of the hoses between servo and engine 1st and if OK then get a vacuum gauge onto the pump to ensure is generating at least 0.5bar of vacuumn with the engine idling
if its the later system where the inlet manifold has a throttle then check for hose leaks too
do you still need help?
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