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Alternating soft and hard pedal when bleeding.


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Around 3 weeks ago the daughter complained that the brakes on her Mk2 Spit were off ( a technical aero space eng description!) so I went down and the rear brakes were seized, I fitted one new 3/4in slave and got a NOS Girlock from a reputable UK supplier, arrived in OZ in a week, after fitting both new slaves and bleeding the brakes the daughter pushing the pedal  she complained every second push of the pedal went to the floor but the alternate pedal was hard! 

At the other end the fluid from the bleed nipple was clear of any air and plenty of new fluid was purged thro, I didn't overhaul the type 12 calipers they were new around 12 years ago, and they apppeared to lock up and release OK.

Over the last couple of days the daughter has operated the brake pedal with the car on axle stands and a hard pedal was evident even after repeated operations, we're going to continue this checking till the weekend then I'll take it a tentative run being very careful, and hopefully build up confidence.

Any ideas anyone, could the caliper piston be releasing too much pushing back into the caliper, giving the long pedal, also I haven't looked at the master there's no sign of any leakage. i  

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sounds to me like the master cylinder played up. The seal that blocks off the return to the reservoir can fail to seal properly so instead of pressuring the circuit fluid just gets pushed into the reservoir with a corresponding slight rise in its level. The second press can improve the sealing so that then the brakes work ok.

I think it could have been a temporary aberation due to the bleeding process as when doing this your not developing the full pressure in the master cylinder to assist with holding the seal closed... 

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There is another similar problem you can get with the rear brakes. If one of the rear hubs is slightly bent it can work with the shues to push the rear cylinder in while driving, thus the first push takes up the fluid and the second has a firm brake.

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4 hours ago, Peter Truman said:

 

Don't know how I've done re the above, but thanks folks for the info, I think we had the master S/S sleeved so a new set of seals should suffice, if not a new master might see if I can get one with more capacity as the existing ain't big only about clutch master capacity

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In my box's of spares I found a larger reservoir (2in dia) Vitesse style brake master cylinder to replace the smaller reservoir of the Spitfire (1.375in dia) it has the same cylinder/piston bore as Spitfire 5/8in. I refurbed this Vitesse style larger reservoir brake master cylinder several years ago & it checks out OK, it's not Girlock but the Aussie PBR brand as fitted as OEM on many Aus and American cars, so there's no worries on it's quality.

Question, I've heard of priming the master cylinder before fitting so as to minimize the potential of air entrainment, what does this procedure consist of?

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just pump like youre emptying the swamp !!!  ..........  well not really 

can you imagine anyone faffing around on the assy line yes dear add a little more 

if you talk to any proper brake man a swift down and slow back is all you need to do 

give the nipple a light nip between strokes if you wish  i always advocate a long bleed tube  and have the dump can

high above the wheels  you can see the air rising and the clear fluid appearing without lying on your back with the glasses upside down

Pete

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