[dmorg1]

Originally Posted by JUDGE3

I was understanding that with manual brakes, you WANT a larger bore master cy.
please explain regarding getting the right bore size. curious due to when I ordered mine, MP brakes had the kit for the swap stating the larger bore. pushing more brake fluid for the manual brake made sense to me. mine are front disc/rear drum and a new replacement type proportioning valve. also will an adjustable proportioning valve help with manual brake?

This is a chart from the 68 service manual. If it is correct the difference in MC bore diameters was not dictated by Manual or Power but by the Disc/Drum and Drum/Drum configurations.


All Disc/Drum combinations for this year, Power and Manual used the larger 1-1/8" bore.





Dave

[wakesupremo]

Thanks again for the replys. Yes I have 4 wheel disc brakes. Willwood rears and stock D52 type callipers on the front. I guess I'll have to try to bleed them again.

[JUDGE3]

wakesupremo not trying to rob your thread, I swapped to manuals (that I prefer) and have a similar symptom. following to learn some info!

good replies, so I checked my order and I do in fact have a 1" bore mc so I am good there. had not seen a chart as posted, did not know the original pwr brake mc bore size now I do. so I have a 1/8" smaller mc now not larger.

as the op stated, I to have bled my brakes so many times I'm supporting the brake fluid manufacturer. I also turned the rear drum adjuster out as far as possible and still be able to install the drum.

I have brakes but just like wakesupremo, if I push hard on the pedal, I can push it all the way to the floor. this would seem to me air in the lines but as stated I can't imagine that being the case?

I swapped my previous 1969 pontiac (I raced) to manual and had a nice firm pedal so I just don't see whats going on with this swap. would a bad (new) proportioning valve cause this you think?

[Lemans64]

Did you bench bleed the master cylinder before installing it? If not it probably still has air in it. The master sits on an uphill slope
which traps the air in the front section of master cylinder. It's just about impossible to get the air out. You almost have to lift the rear of
car up to level master to get the air out. Just a thought.

[dmorg1]

If you are confidant that all of the air has been purged from the system and you still have a poor pedal it may be possible that the MC was not assembled properly.

I recently posted this picture in another brake thread. This is the piston from a new out of the box MC that I pulled apart in order to machine for a residual valve.

There is no way that this seal would hold pressure.









Dave

[Scarebird]

Dmorg: that is interesting. I did not think Pontiac offered disc in A bodies without power boost. It seems that the 4 piston calipers had a different spec versus the 1 piston units we all know and love. Note NAPA shows BOTH in 1967 and 68-69 Firebird as having 1-1/8" bore. I think they goofed as the 1969 had the one piston units.

[wakesupremo]

I have brakes. At last lots more bleeding and finally I have brakes. The pedal doesn't fully return though still. It doesn't return enough to switch off the brake lights!!!

[dmorg1]

Wakesupremo,


A few things that may or may not help with your situation.


This statement from your first post concerning the spring

"never had one, that'll explain why the pedal has been slow returning!!"

suggests that possibly the MC pistons may be gummed up and not returning completely.


As Scarebird stated: The spring is not there to return the pedal as much as keep it from rattling.
Is true, I believe some models don't even use springs.


How much end play is there in the pushrod?


Disconnect the clevis from the pedal and check for excessive end play. This would be the distance the pushrod can move while trapped between the pushrod retainer and piston.


Dave

[wakesupremo]

Thanks again dmorg1. I assume that freeplay is determined by lifting the pedal to its uppermost point and measuring between. As I haven't really had any pedal as such its a long way. Now that I have brakes can I adjust the rod length to push the pedal back against the M/C piston? I have at least 1 1/2 inches of adjustment on the clevis attachment.

[dmorg1]

Wakesupremo,

I am suspecting that there is something funky going on with the pushrod.

The adjustment at the clevis and pushrod connection point is only a means to adjust the pedal height.

In order to check the end play you will have to disconnect the clevis from the pedal and check the movement.

You can do this in the vehicle, once disconnected from the pedal just move the pushrod from front to rear with your hand and kinda gage the movement.

The end play should be minimal. I have never seen a Pontiac spec. for end play but the few that I have checked were in the .015"-.025" range. MBM recommends no more than 1/32"






Dave

[wakesupremo]

Thanks again. I think we both mean the same thing just different ways of explaining it.
My pedal won't come back far enough to switch off the brake lights. The pushrod has at least 1 " of rod through the clevis almost touching the rear of the pedal but not quite. I can extend the pushrod against the resistance of the M/C piston to lift the pedal up towards the brake light switch but, obviously, if I am using the M/C piston as the 'push point' it will alleviate any freeplay.

[Skip Fix]

I had a new out of box 1" Vette style MC bad, so it happens.

[dmorg1]

Adjusting at the clevis will not eliminate the end play of the pushrod portion trapped between the MC piston and the Pushrod Retainer. It will only adjust the pedal position.

The 68 service manual is showing a 6-3/4" reference distance between the pedal pad and the floor for Manual brakes and a 4-3/32" minimum distance for Power brakes. I am not sure if 69 uses this same distance. I dont have a 69 manual.

Also Manual will use the upper pedal hole to mount the clevis and Power use the lower hole

With the pedal adjusted to the proper height there will be more than enough adjustment at the switch.

If you cannot make contact to activate the switch at the proper pedal height then there is another issue.

When you swapped to the 4 disc MC was this MC capable of being used on both Manual and Power systems?


Dave

[wakesupremo]

Thanks again dmorg1. I took a corvette 4 wheel power disc master cylinder off and replaced with a brand new corvette non power master cylinder. As far as I'm aware the only difference would be the recess in the piston for the pushrod. The power one only has a small dimple whereas the non power one has a deep slot. I will get back under there tonight and alter the pedal height with the adjustment I have and see what that does. Thanks again for your invaluable help.

[wakesupremo]

I have tried extending the pushrod. That pushes the piston into the master cylinder rather than push the pedal back. Im guessing that means that there is still air somewhere as there should be more resistance?

[dmorg1]

Originally Posted by wakesupremo

Thanks again dmorg1. I took a corvette 4 wheel power disc master cylinder off and replaced with a brand new corvette non power master cylinder. As far as I'm aware the only difference would be the recess in the piston for the pushrod. The power one only has a small dimple whereas the non power one has a deep slot. I will get back under there tonight and alter the pedal height with the adjustment I have and see what that does. Thanks again for your invaluable help.

This is opposite of what I have found on my 68 A Bodies but it really doesn't matter because adapters/plugs are available to allow for interchanging.

And yes, I also believe that is the only difference.

It is most important to achieve the proper gap/endplay whether using or not using a booster or the adapter/plug.

This is what I suggest:

1. Disconnect the clevis from the pedal.

2. Confirm that you have the correct gap/endplay between the MC piston and pushrod, adjust if necessary.

3. Cut a block of wood or find something that is equal to the proper pedal height and support the pedal. (Be sure the pedal pivot is in the correct hole)

with the pedal still disconnected from the clevis and at the proper height there should be enough adjustment at the switch to activate it. If not then you have mismatched components that you will have to sort out in order to correct.

4. Once you have the switch activating at the proper pedal height go ahead and adjust the pushrod at the clevis and attach to the pedal. (be sure to maintain the proper pedal height.)


Hope this helps.








Dave

[wakesupremo]

Thanks again dmorg1. I'm still uncertain how you determine the end play, My pushrod just fits in the hole so I can pull it in and out as much as I want. As I previously said, I was expecting to be able to sort that by pushing the pedal up against the light switch and then backing it off a turn, however that doesn't work as it just pushes the M/C piston in under the weight of the pedal. the system that you have described is basically what I have done. The weight of the pedal pushes it down though as if there isn't enough resistance in the master cylinder. I am now doubting the proportioning valve and actually wonder if i need it as I have 4 wheel disc brakes!!!

[dmorg1]

Originally Posted by wakesupremo

Thanks again dmorg1. I'm still uncertain how you determine the end play, My pushrod just fits in the hole so I can pull it in and out as much as I want.

That is not a good situation.


With the Manual system, the pushrod should be trapped between the piston and the pushrod retainer. It sounds like you have no pushrod retainer or a pushrod with no flange.



In the pic below, the circled end of the pushrod should be trapped between the MC piston and the retainer. The retainer is the round part (lower center) that snaps onto the nose at the rear of the MC after the threaded end of pushrod has been placed through it.

it just pushes the M/C piston in under the weight of the pedal. the system that you have described is basically what I have done. The weight of the pedal pushes it down though as if there isn't enough resistance in the master cylinder.


Even if the MC were empty and had no lines attached it would take much more than the weight of a hanging pedal to move the piston

["QUICK-SILVER"]

Originally Posted by wakesupremo

I have tried extending the pushrod. That pushes the piston into the master cylinder rather than push the pedal back. Im guessing that means that there is still air somewhere as there should be more resistance?

If the master cylinder was bone dry, the piston return springs should be plenty strong enough to hold the pedal up instead of the weight of the pedal pushing the piston in.

While bleeding brakes; If the front piston didn't return far enough to expose the inlet hole for fluid intake... May have created a vacuum that's holding the front piston forward. That would make for a low pedal and lack of return spring pressure.

Are all your caliper bleeder valves in the UP position?

You've got something strange going on here or the MC is defective/got assembled wrong.

Clay

[wakesupremo]

Thanks again for the replys. I definitely don't have all that you have shown dmorg1. Thats probably where the issue lies I have no retainer or even the rubber boot type thing!!!!