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mg_zt_t
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My R-Reg Rover 620ti failed its MoT on very poor handbrake operation two weeks ago. I planned to replace both rear Brake Calipers with a pair I had DIY refurbished. Whilst bleeding the two newly fitted Calipers, took the opportunity to bleed all four Brake Calipers pushing through the old discoloured Brake Fluid replacing it with fresh crystal clear fluid.

Job done .... er... no. Very low Brake Pedal ... :(

This also happened last year when we bled out all the old fluid from my son's high mileage 620ti. He had a 'soft' pedal following a lap of the Nurburgring with four friends on board where he used the brakes quite a lot. Yes, having bled all four his pedal was low but still had braking. Even so not ideal.

So I searched the web and found that it can be a problem when replacing the whole of the fluid in the Braking System. Certainly with our two 620ti and maybe other ABS systems.

I also learned that it is necessary to "cycle" the ABS Modulator to exclude any air reaching the valves in the Modulator after a fluid change. I believe pro-equipment is used in the workshops. Obviously I do not have access to that kit.

I also learned that by driving the car on rough grassy ground and applying the brakes to get the ABS working overtime, that is the way I test check ABS operation on any used car I buy anyway, can purge away any small quantities of air in the Modulator.

With no Tax or MoT, could not easily arrange that but with the car up on four axle stands and all wheels off, the same modulator "purge" result can be achieved in the following way.

With engine idling, spin the rear offside wheel manually as fast as you can then whilst the wheel is spinning round, apply the brake pedal firmly. With just the one wheel moving the ABS will be deployed and you will feel the pulses under your foot briefly on the brake pedal. Repeat with other rear wheel spinning. I did this but, did not feel those pulses underfoot when I applied the brake pedal firmly.

I then turned my attention to the front axles. I put the two wheels back on and wedged some wood underneath the offside ( driver's side ) wheel to stop it rotating. I then started the engine and engaged first gear gently lifting off the clutch. This set the free nearside wheel rotating. I gently raised the engine revs to 2,000 rpm and lifted off the throttle pedal and immediately firmly applied the Footbrake Pedal. I felt those ABS pulses very briefly for a second or so until the engine stalled with my foot still firmly on the brake pedal.

I was delighted to find I had a nice high and firm Footbrake Pedal after the process. The brakes are now superb on this old car and the free MoT retest was completed with no advisories.

We will repeat that process with my son's 620ti currently stored on SORN to regain that 'firm' pedal when he next wants to use that car. Probably at the end of the summer.
 

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That's the reason I always prefer to vacuum bleed when possible. No risk of air getting in so long as you keep the master topped up.
 
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