bleeding brakes

Moderators: 330GT, abrent

cory
Posts: 227
Joined: Thu Apr 14, 2011 7:34 pm

Re: bleeding brakes

Post by cory »

It was my understanding that within the piston itself there is a small spring and this small spring works with the pin like rod that it fits onto when the piston is the bore. My front caliper spring has a break in it and I am told that is why I can't get it to release all the way. Its not grabbing but maintaining enough pressure so that the free wheel spin is not like its opposite wheel. As a remedy I honed the bore a touch thinking maybe the new bore tolerances were too tight to the new seals. No luck...same result. Please correct me if I'm off....
Cory
BobA
Posts: 103
Joined: Tue Nov 12, 2013 7:36 pm

Re: bleeding brakes

Post by BobA »

John: Check the small hole behind the MC o-ring which open to the reservior. That was what caused my SWB to hold pressure. The o-ring had swollen and the piston was not able to clear the orifice. We took a .020" cut off the piston. Probably not the best procedure, but it worked. Bob
John Vardanian
Posts: 1906
Joined: Wed Sep 04, 2002 12:13 am
Location: San Francisco Area

Re: bleeding brakes

Post by John Vardanian »

Thanks Bob and Kerry for the explanation. I fixed the problem. The replacement front seal I had bought for the EQ valve was a little thinner than the original, so I had cut out a filler from rubber material to take up the slack. Apparently that was interfering with the operation of the EQ valve. I removed the extra piece.

The issue now is that there is some slight drag on all four wheels, not enough to keep the car from rolling, but enough to take effort to turn the splined hub with your wrist. The wheel cylinders are all lined with stainless sleeves and every other part in the system is either new or refurbished. The system is bled and the pedal is incredibly firm. From the explanation Kerry gave on the method of wheel cylinder retraction, I can see why the pistons may not retract all the way the way the drum brake shoes retract with the spring assist. So, maybe things will loosen up with use.

Thank you all for your advice.

john
PF Coupe
BobA
Posts: 103
Joined: Tue Nov 12, 2013 7:36 pm

Re: bleeding brakes

Post by BobA »

I have always been taught that the little retractor pins and springs are the mechanism which causes the pads to retract when the pressure is released. The other mechanism is the slight runout of the rotor moving the pistons back into the caliper cylinders.
John Vardanian
Posts: 1906
Joined: Wed Sep 04, 2002 12:13 am
Location: San Francisco Area

Re: bleeding brakes

Post by John Vardanian »

But how Bob? I always wondered the purpose of those pins myself.

john
PF Coupe
BobA
Posts: 103
Joined: Tue Nov 12, 2013 7:36 pm

Re: bleeding brakes

Post by BobA »

John: I found this online. Maybe it will do the trick. http://mgaguru.com/mgtech/twincam/tc207.htm
cory
Posts: 227
Joined: Thu Apr 14, 2011 7:34 pm

Re: bleeding brakes

Post by cory »

Great post BobA!!
Cory
BobA
Posts: 103
Joined: Tue Nov 12, 2013 7:36 pm

Re: bleeding brakes

Post by BobA »

Mystery Solved?
John Vardanian
Posts: 1906
Joined: Wed Sep 04, 2002 12:13 am
Location: San Francisco Area

Re: bleeding brakes

Post by John Vardanian »

Bob, thanks for posting this, it saved my life. The last sentence of the second paragraph turned on the bulb in my head and I spotted my problem right away. Damned amateurs!

john
PF Coupe
BobA
Posts: 103
Joined: Tue Nov 12, 2013 7:36 pm

Re: bleeding brakes

Post by BobA »

Can someone please advise me of the purpose of the small hole (Which I cannot find) in the booster cylinder. The only purpose I can think of would to allow air to bleed out, but that should be accomplished by the clearance between the rod on the end of the piston and the casting. Bob A
John Vardanian
Posts: 1906
Joined: Wed Sep 04, 2002 12:13 am
Location: San Francisco Area

Re: bleeding brakes

Post by John Vardanian »

Bob, where is this hole exactly?

john
PF Coupe
250GT
Posts: 968
Joined: Wed Oct 15, 2008 7:41 am
Location: germany/holland

Re: bleeding brakes

Post by 250GT »

Can not been overseen John with good eyes ,but its maybe only 10 mu.
Its located under the bracket from the chassis ,see my earlier post here.
ciao
C.
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John Vardanian
Posts: 1906
Joined: Wed Sep 04, 2002 12:13 am
Location: San Francisco Area

Re: bleeding brakes

Post by John Vardanian »

Thanks Cornelis. Is the one in your picture the longer type that goes on the earlier cars?

john
PF Coupe
John Vardanian
Posts: 1906
Joined: Wed Sep 04, 2002 12:13 am
Location: San Francisco Area

Re: bleeding brakes

Post by John Vardanian »

Cornelis, I see that this is the cast iron type that goes on the earlier cars, like my PF coupe. The later cars have smaller cylinder made of cast aluminum.

john
PF Coupe
BobA
Posts: 103
Joined: Tue Nov 12, 2013 7:36 pm

Re: bleeding brakes

Post by BobA »

My question was "What is the purpose of the small hole"? It must have something to do with air pressure, but since the shaft at the end of the cylinder is not really air tight, I wonder that the hole is for. I have found that usually there are not machining features put in parts without there being a reason. Bob A
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