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Discussion Starter #1
Hi,
Can anyone please help me? I have a 1973 105 GT Junior, with floor mounted pedals and twin servos.
Last summer (2019), I replaced my brake master cylinder, the brakes worked as expected.
Then, after a some weeks my brake pedal travel increased, but the pedal remains firm (not spongy). I noticed that my front brakes were locking up more that I would normally expect.
So, I took the car to a MOT test station and put it on the rolling road brake tester. My front brakes are perfect, but I am not getting any braking from the rear brakes.
Does any one have any ideas?

Thanks
Dino
 

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Does your car have the rear brake bias modulator? I suppose it could be corroded in a manor that doesn't all fluid to the rear.
 

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Does your car have the rear brake bias modulator? I suppose it could be corroded
Yes. Or the rear flex hose could be old, causing it to swell internally and stop passing fluid. But, when that happens, usually the rear brakes lock up, rather than fail to work. Or it could be something in the master cylinder - I know it is fairly new, but....
 

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Discussion Starter #4
Yes. Or the rear flex hose could be old, causing it to swell internally and stop passing fluid. But, when that happens, usually the rear brakes lock up, rather than fail to work. Or it could be something in the master cylinder - I know it is fairly new, but....
The rear flexi line is only 10 years old, and looks fine.
I did bleed the brakes and the fluid came through as you would expect, so no obvious signs of blockage there in the flexi line.
I initially started to worry about the brakes when the pedal travel increased, so it would be good to understand why, as that should take us to the root cause.
I do have a brake pressure regulator valve, but I can not reason a failure mode where all fluid flow to the calipers is restricted. I am loath to change it unless I am certain that it is the root cause.
Any ideas?
Thanks
Dino.
 

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Just because the master is new don’t rule that out, one of the seals could have failed. The late great Papafam taught me to always disassemble any new master, many times during the manufacturing process the bores are not cleaned properly from honing. Grit particles are left inside and wears the seal till it fails. I purchased a slave from Classicalfa or Alfaholics’s, not sure which but when I dissembled it to clean it this is what I found! Always wondered what grief that would have caused.
 

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Hi Dino

That period GT (like mine) has the dual circuit brakes for safety, so that one circuit should always work even if one fails. But instead of splitting diagonally like every other manufacturer Alfa went for a front/rear circuit split. As such it probably indicates something wrong with the rear hydraulic circuit.

Servo would be top candidate for suspicion. Also as other mentioned brake proportioning valve to rear. Servos are rebuildable or can be bought on service exchange basis in the UK. Also if fluid level is dropping maybe a small leak in the rear system so that pressure is simply not building up. I know there have been a few issues with brake light pressure sensor switch sticking and heating up the bake fluid with failure symptoms. You'd be able to feel it hot.

The front circuit sounds like it is running fine.

If its been like that for a while the situation could be compounded by rear brakes which have not worked properly (or at all) for years and so may have partially seized. It may be the calipers need a clean up and service to free them off as well.

Those dual servo systems are a pain to work with. Some users dispense with both servos but the pedal pressure required is obviously higher and you may need to upgrade front discs, fit new Master cylinder etc.
 

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Discussion Starter #9
HI Bertie,
I am interested in understanding how the servos could fail? Would failure in one servo increase brake pedal travel? There is no brake fluid loss, so what do I need to look out for or check?
So many questions, but I have never had a problem like tis before.
Many thanks,
Dino
 

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I'd Disconnect the vacuum line to the rear servo and see what happens, just a bit of eleminiation
 
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