1970 Corvette 350 cu. in. brakes

I have been messing about with these brakes for months on and off. I
postede in May time about the difficulty of getting fluid through to
the back calipers. See he
http://groups.google.co.uk/group/alt...8 023a0e83272

I have spent over 1000 dollars on parts so far and I still have a
problem.

Four new calipers, the complete set of bits to do the handbrake, new
pipes, new pads, lots of odds and ends and so much time that my head
spins just thinking about it.

I thought it must be the pressure differential sensing valve, but I
took it out and pushed the spool back and forth against the springs
and the contact made at the appropriate time AND air could get through
both ways, i.e. front and rear when it was at rest. I tried bleeding
the front brakes with a vacuum pipe on the caliper bleeding nipples. I
made up a special bleed nipple that did not seal but let fluid through
all the time. With a big reservoir above the master cylinder and a
glass jug between the nipple and my vacuum pump, everything looked
fine and I got lots of clear fluid going through. This should have
not actuated the pressure valve at all.

However when I tried the same on the rear calipers, it was as if the
valve had sealed again, as no fluid could be sucked through. It was
too late to do any more, today, but what do you think about that? As
far as I know there is nothing else that could stop the flow. Or
could it even be the set up of the master cylinder clearances. It WAS
working and when I took it all apart it looked in perfect condition so
I put it all back as it was. Cleaned out with new DOT 4 fluid, of
course.

I am glad to say the lights and wipers and associated vacuum circuits
are working perfectly now too. I have not really got that much more
to do.

This car has got to go out of my workshop very soon as I want to build
a bio-diesel processor exactly where it is now. I has been jacked up
for months.

Anyone got any ideas? Please. Regards George.

"George" > wrote in message
...
>I have been messing about with these brakes for months on and off. I
> postede in May time about the difficulty of getting fluid through to
> the back calipers. See he
> http://groups.google.co.uk/group/alt...8 023a0e83272
>
> I have spent over 1000 dollars on parts so far and I still have a
> problem.
>
> Four new calipers, the complete set of bits to do the handbrake, new
> pipes, new pads, lots of odds and ends and so much time that my head
> spins just thinking about it.
>
> I thought it must be the pressure differential sensing valve, but I
> took it out and pushed the spool back and forth against the springs
> and the contact made at the appropriate time AND air could get
> through
> both ways, i.e. front and rear when it was at rest. I tried
> bleeding
> the front brakes with a vacuum pipe on the caliper bleeding nipples.
> I
> made up a special bleed nipple that did not seal but let fluid
> through
> all the time. With a big reservoir above the master cylinder and a
> glass jug between the nipple and my vacuum pump, everything looked
> fine and I got lots of clear fluid going through. This should have
> not actuated the pressure valve at all.
>
> However when I tried the same on the rear calipers, it was as if the
> valve had sealed again, as no fluid could be sucked through. It was
> too late to do any more, today, but what do you think about that? As
> far as I know there is nothing else that could stop the flow. Or
> could it even be the set up of the master cylinder clearances. It
> WAS
> working and when I took it all apart it looked in perfect condition
> so
> I put it all back as it was. Cleaned out with new DOT 4 fluid, of
> course.
>
> I am glad to say the lights and wipers and associated vacuum
> circuits
> are working perfectly now too. I have not really got that much more
> to do.
>
> This car has got to go out of my workshop very soon as I want to
> build
> a bio-diesel processor exactly where it is now. I has been jacked up
> for months.
>
> Anyone got any ideas? Please. Regards George.
>

Is it possible you have the pistons in the back brake calipers
bottomed out? Did you put on new hoses?

On 12 Jul, 23:19, "Dad" > wrote:
> "George" > wrote in message
>
> ...
>
>
>
>
>
> >I have been messing about with these brakes for months on and off. I
> > postede in May time about the difficulty of getting fluid through to
> > the back calipers. * See he
> >http://groups.google.co.uk/group/alt...wse_thread/thr...
>
> > I have spent over 1000 dollars on parts so far and I still have a
> > problem.
>
> > Four new calipers, the complete set of bits to do the handbrake, new
> > pipes, new pads, lots of odds and ends and so much time that my head
> > spins just thinking about it.
>
> > I thought it must be the pressure differential sensing valve, but I
> > took it out and pushed the spool back and forth against the springs
> > and the contact made at the appropriate time AND air could get
> > through
> > both ways, i.e. front and rear when it was at rest. *I tried
> > bleeding
> > the front brakes with a vacuum pipe on the caliper bleeding nipples.
> > I
> > made up a special bleed nipple that did not seal but let fluid
> > through
> > all the time. *With a big reservoir above the master cylinder and a
> > glass jug between the nipple and my vacuum pump, everything looked
> > fine and I got lots of clear fluid going through. *This should have
> > not actuated the pressure valve at all.
>
> > However when I tried the same on the rear calipers, it was as if the
> > valve had sealed again, as no fluid could be sucked through. *It was
> > too late to do any more, today, but what do you think about that? As
> > far as I know there is nothing else that could stop the flow. *Or
> > could it even be the set up of the master cylinder clearances. *It
> > WAS
> > working and when I took it all apart it looked in perfect condition
> > so
> > I put it all back as it was. *Cleaned out with new DOT 4 fluid, of
> > course.
>
> > I am glad to say the lights and wipers and associated vacuum
> > circuits
> > are working perfectly now too. *I have not really got that much more
> > to do.
>
> > This car has got to go out of my workshop very soon as I want to
> > build
> > a bio-diesel processor exactly where it is now. I has been jacked up
> > for months.
>
> > Anyone got any ideas? *Please. * Regards George.
>
> Is it possible you have the pistons in the back brake calipers
> bottomed out? Did you put on new hoses?- Hide quoted text -
>
> - Show quoted text -

Hi Dad, I'm not sure what bottomed out means. The calipers were
brand new and the same style etc. as the ones that came out. I did
not change any of the pipes as yet as they are not bad and I want to
see it work before I change anything else. All the oil that came out
of various places was old, but clean, so I don't expect any
blockages. Even if the pistons are at the bottom of the bores, the
fluid would still get in. No, the more I think about it there is
something very basic stopping the fluid going through and my reasoning
is at fault. I shall take the pipes off the master cylinder and the
pressure differential again and see where the actual blocking is
taking place. I may take them both to the agent and buy whatever he
says is faulty. The annoying thing is that they all seem OK but the
front works perfectly and I can't get a drop through to the back end.

I may be too close to the problem and can't see the wood for all the
trees!

Regards George.

On 13 Jul, 21:56, George > wrote:
> On 12 Jul, 23:19, "Dad" > wrote:
>
>
>
>
>
> > "George" > wrote in message
>
> ....
>
> > >I have been messing about with these brakes for months on and off. I
> > > postede in May time about the difficulty of getting fluid through to
> > > the back calipers. * See he
> > >http://groups.google.co.uk/group/alt...wse_thread/thr....
>
> > > I have spent over 1000 dollars on parts so far and I still have a
> > > problem.
>
> > > Four new calipers, the complete set of bits to do the handbrake, new
> > > pipes, new pads, lots of odds and ends and so much time that my head
> > > spins just thinking about it.
>
> > > I thought it must be the pressure differential sensing valve, but I
> > > took it out and pushed the spool back and forth against the springs
> > > and the contact made at the appropriate time AND air could get
> > > through
> > > both ways, i.e. front and rear when it was at rest. *I tried
> > > bleeding
> > > the front brakes with a vacuum pipe on the caliper bleeding nipples.
> > > I
> > > made up a special bleed nipple that did not seal but let fluid
> > > through
> > > all the time. *With a big reservoir above the master cylinder and a
> > > glass jug between the nipple and my vacuum pump, everything looked
> > > fine and I got lots of clear fluid going through. *This should have
> > > not actuated the pressure valve at all.
>
> > > However when I tried the same on the rear calipers, it was as if the
> > > valve had sealed again, as no fluid could be sucked through. *It was
> > > too late to do any more, today, but what do you think about that? As
> > > far as I know there is nothing else that could stop the flow. *Or
> > > could it even be the set up of the master cylinder clearances. *It
> > > WAS
> > > working and when I took it all apart it looked in perfect condition
> > > so
> > > I put it all back as it was. *Cleaned out with new DOT 4 fluid, of
> > > course.
>
> > > I am glad to say the lights and wipers and associated vacuum
> > > circuits
> > > are working perfectly now too. *I have not really got that much more
> > > to do.
>
> > > This car has got to go out of my workshop very soon as I want to
> > > build
> > > a bio-diesel processor exactly where it is now. I has been jacked up
> > > for months.
>
> > > Anyone got any ideas? *Please. * Regards George.
>
> > Is it possible you have the pistons in the back brake calipers
> > bottomed out? Did you put on new hoses?- Hide quoted text -
>
> > - Show quoted text -
>
> Hi Dad, * I'm not sure what bottomed out means. The calipers were
> brand new and the same style etc. as the ones that came out. *I did
> not change any of the pipes as yet as they are not bad and I want to
> see it work before I change anything else. All the oil that came out
> of various places was old, but clean, so I don't expect any
> blockages. *Even if the pistons are at the bottom of the bores, the
> fluid would still get in. *No, the more I think about it there is
> something very basic stopping the fluid going through and my reasoning
> is at fault. *I shall take the pipes off the master cylinder and the
> pressure differential again and see where the actual blocking is
> taking place. *I may take them both to the agent and buy whatever he
> says is faulty. * The annoying thing is that they all seem OK *but the
> front works perfectly and I can't get a drop through to the back end.
>
> I may be too close to the problem and can't see the wood for all the
> trees!
>
> Regards George.- Hide quoted text -
>
> - Show quoted text -

See? Do not assume anything and start from the simple facts. I assumed
that because I read about the pressure differential valve could block
off a failing front or back system. So that must be it - isn't it?
NO Twit! Another reason that no fluid is getting to either rear
caliper just could be two pipes blocked! And so it was.

The flexible pipes at the rear were both solid and I could not even
hammer a thin brazing rod through. When I replaced these everything
suddenly went right. With my suction pump, I did not even have to
prime the master cylinder. The fluid went through so fast I shall
have to put a tap on it to regulate the vacuum. I am really looking
forward to doing it tomorrow now.

Thanks for listening. Nothing like talking to yourself to get an
application of logic.

Regards George.

"George" > wrote in message
...
Snip

Rememeber me asking if you had changed the hoses? Now you know why I
ask.

>
> > Is it possible you have the pistons in the back brake calipers
> > bottomed out? Did you put on new hoses?- Hide quoted text -
>
> > - Show quoted text -
>
> Hi Dad, I'm not sure what bottomed out means. The calipers were
> brand new and the same style etc. as the ones that came out. I did
> not change any of the pipes as yet as they are not bad and I want to
> see it work before I change anything else. All the oil that came out
> of various places was old, but clean, so I don't expect any
> blockages. Even if the pistons are at the bottom of the bores, the
> fluid would still get in. No, the more I think about it there is
> something very basic stopping the fluid going through and my
> reasoning
> is at fault. I shall take the pipes off the master cylinder and the
> pressure differential again and see where the actual blocking is
> taking place. I may take them both to the agent and buy whatever he
> says is faulty. The annoying thing is that they all seem OK but the
> front works perfectly and I can't get a drop through to the back
> end.
>
> I may be too close to the problem and can't see the wood for all the
> trees!
>
> Regards George.- Hide quoted text -
>
> - Show quoted text -

See? Do not assume anything and start from the simple facts. I assumed
that because I read about the pressure differential valve could block
off a failing front or back system. So that must be it - isn't it?
NO Twit! Another reason that no fluid is getting to either rear
caliper just could be two pipes blocked! And so it was.

The flexible pipes at the rear were both solid and I could not even
hammer a thin brazing rod through. When I replaced these everything
suddenly went right. With my suction pump, I did not even have to
prime the master cylinder. The fluid went through so fast I shall
have to put a tap on it to regulate the vacuum. I am really looking
forward to doing it tomorrow now.

Thanks for listening. Nothing like talking to yourself to get an
application of logic.

Regards George.

Amazing that both hoses were blocked. You would expect only one to be
blocked and if you have the same problem on both rears, then the master
cylinder or 'proportioning' valve are the only common elements.

Glad you got it fixed.


"Dad" > wrote in message
.. .
>
> "George" > wrote in message
> ...
> Snip
>
> Rememeber me asking if you had changed the hoses? Now you know why I ask.
>
>>
>> > Is it possible you have the pistons in the back brake calipers
>> > bottomed out? Did you put on new hoses?- Hide quoted text -
>>
>> > - Show quoted text -
>>
>> Hi Dad, I'm not sure what bottomed out means. The calipers were
>> brand new and the same style etc. as the ones that came out. I did
>> not change any of the pipes as yet as they are not bad and I want to
>> see it work before I change anything else. All the oil that came out
>> of various places was old, but clean, so I don't expect any
>> blockages. Even if the pistons are at the bottom of the bores, the
>> fluid would still get in. No, the more I think about it there is
>> something very basic stopping the fluid going through and my reasoning
>> is at fault. I shall take the pipes off the master cylinder and the
>> pressure differential again and see where the actual blocking is
>> taking place. I may take them both to the agent and buy whatever he
>> says is faulty. The annoying thing is that they all seem OK but the
>> front works perfectly and I can't get a drop through to the back end.
>>
>> I may be too close to the problem and can't see the wood for all the
>> trees!
>>
>> Regards George.- Hide quoted text -
>>
>> - Show quoted text -
>
> See? Do not assume anything and start from the simple facts. I assumed
> that because I read about the pressure differential valve could block
> off a failing front or back system. So that must be it - isn't it?
> NO Twit! Another reason that no fluid is getting to either rear
> caliper just could be two pipes blocked! And so it was.
>
> The flexible pipes at the rear were both solid and I could not even
> hammer a thin brazing rod through. When I replaced these everything
> suddenly went right. With my suction pump, I did not even have to
> prime the master cylinder. The fluid went through so fast I shall
> have to put a tap on it to regulate the vacuum. I am really looking
> forward to doing it tomorrow now.
>
> Thanks for listening. Nothing like talking to yourself to get an
> application of logic.
>
> Regards George.
>