275GTB brake help

[tyang]

O.K. Guys, I'm baffled, and I need some help.

I'm project managing 275GTB restoration and we've run into a problem with the brakes. The braking system was working fine on this car before the restoration, and was completely disassembled by the shop doing the restoration. I was not involved with this car during the disassembly process, so I'm taking their word for it.

When the M/C was re-installed with new calipers, we found a leaking rear seal on the M/C. It was decided by the shop to send it to a rebuilder in VA. (not my first choice) Upon return, the M/C was reinstalled, and we began the bleeding procedure, but I could not get pressure to the rear brakes. We tried gravity bleeding, pressure bleeding, and vacuum bleeding to no avail. The M/C was bench bled twice with fluid flowing from both circuits, but when it's installed on the car, we get no pressure to the rear circuit. I've adjusted free play to see if anything changes, but nothing.

The strange thing is when the pedal is at rest, fluid flows freely to the rear calipers when the bleeder is open, but the second you apply pedal pressure, the fluid stops flowing, and there is no pressure at the caliper.

Another time, we tried bleeding only the rear calipers where no fluid was in the front circuit. At the last 1/4 inch of stroke, I could feel the pedal moving fluid to the rear circuit. The guy at the rear bleeder screw found fluid moving to the rear calipers. We bled both rear calipers with no pedal feel until the very end of the travel, and stopped when the air was gone. We then moved to the front circuit, and bled it normally, with good pedal feel, and a falling pedal when the bleeder was cracked. Once all the air was out of the front system, we went to the back and found no pressure or fluid coming out of the bleeder!? With a hard pedal, it seemed there was not enough travel to move fluid to the rear circuit. Not that it is right, but it seemed the M/C would only put pressure to the rear circuit when the pushrod is at the bottom of the stroke.

Before I blame the M/C restorer, I want to make sure it's something I'm not missing. Any thoughts would be greatly appreciated!

Tom

[DWR46]

Tom: The original 275 Master Cyliinders are some of the most difficult masters to get right that I have ever worked on. They are not easy to take apart or put together. Also the current seals are usually of a slightly different thickness, causiing the compensation port to be sometimes not uncovered with the pedal at rest. But this does not sound like your problem. I would put the master on the bench and plumb a brake pressure gauge into both the front and rear ports. See if you are getting pressure directly out of the master.

I suspect the master is not assembled correctly. You will need to take it apart and compare what you have with the shop manual.

Another thought is "is it an original master"? This would not affect your problem but it could change the cause of what may be going on.

[zac]

Hi Tom,

can you post a picture of the master? if its the one I think it is I think I know whats going on. I have a real good parts blow up of the master cylinders and there are a few mistakes that can be made in the assembly process as well as if they sleeved it and didn't fully remove any burrs on the holes.

Zac

[tyang]

Thanks guys for the input.

Zac,

I won't be able to take a picture of the M/C until monday, but I can describe that it is the early M/C with the black booster. The inlet ports are vertical, and outlet ports are at an angle. Does that help describe the M/C?

Dyke,

The sliding piston for the second circuit is not so simple. I sure hope the rebuilder didn't do something irreversible.

Tom

[zac]

Does the master have the little insert at the bottom of the outlet port for the rear that keeps the secondary piston for the rear circuit located? It is possible when they put it back together they could have left that out or located the rear circuit piston incorrectly and when they installed the locator it scratched the piston causing it to hang up. It is also possible that they got the order of one of the spring-washer-seal combos off that would cause it to do funny things when the front circuit pressurizes. I am not at the shop today but I can send you the blow up in the morning when I do go there. The main thing I try to keep in mind is that in reality brakes are very simple and either they move fluid or don't, don't over think it. If it will not bleed out easily and doesn't feel right go with your first instinct and go back over it, it will not just fix itself:)

Zac

[tyang]

Hi Zac,

That's what's so frustrating. The shop that's doing the restoration wants to make absolute sure it's not something they did before sending the M/C back. My opinion is to send it back because I suspect the rebuilder.

My question is does the length of either push rod should not affect the rear circuit as strangely as it has? I've tried all different lengths to see if there was any change, and the same problem arises.

We've been looking at the parts book and shop manual, but do you have something more detailed?

Thanks!

TOm

[zac]

Again I there are several versions and I have been in 250/Dino mode lately so my memory needs some refreshing. From what I recall the push rod has nothing to do with the rear circuit. Thats why it is critical to get the front all bled out first than move to the rears, the front circuit is pressed on mechanically by the pushrod/booster/pedal but the rear is actuated by the front brake pressure.

What I did see happen on one of my customers GTCs was that they replaced the master with a 308 one and left a way to long pushrod in that bottomed out making it feel like a firm brake pedal only to have the car run right into the back of the first car it had to stop hard behind, needless to say customer was not happy and neither was the nose of his car. Brakes are critical advise your customer that it may take a couple more days to get him driving the car but it is not worth the rush, in the long run he will be much happier to have a safe car to drive.

Zac

[Yale]

White Post strikes again!

[tyang]

Again I there are several versions and I have been in 250/Dino mode lately so my memory needs some refreshing. From what I recall the push rod has nothing to do with the rear circuit. Thats why it is critical to get the front all bled out first than move to the rears, the front circuit is pressed on mechanically by the pushrod/booster/pedal but the rear is actuated by the front brake pressure.

What I did see happen on one of my customers GTCs was that they replaced the master with a 308 one and left a way to long pushrod in that bottomed out making it feel like a firm brake pedal only to have the car run right into the back of the first car it had to stop hard behind, needless to say customer was not happy and neither was the nose of his car. Brakes are critical advise your customer that it may take a couple more days to get him driving the car but it is not worth the rush, in the long run he will be much happier to have a safe car to drive.

Zac

Hi Zac,

I think what you said

Thats why it is critical to get the front all bled out first than move to the rears, the front circuit is pressed on mechanically by the pushrod/booster/pedal but the rear is actuated by the front brake pressure

is perhaps a key to my problem.

On my initial post, I wrote:

Another time, we tried bleeding only the rear calipers where no fluid was in the front circuit. At the last 1/4 inch of stroke, I could feel the pedal moving fluid to the rear circuit. The guy at the rear bleeder screw found fluid moving to the rear calipers. We bled both rear calipers with no pedal feel until the very end of the travel, and stopped when the air was gone. We then moved to the front circuit, and bled it normally, with good pedal feel, and a falling pedal when the bleeder was cracked. Once all the air was out of the front system, we went to the back and found no pressure or fluid coming out of the bleeder!? With a hard pedal, it seemed there was not enough travel to move fluid to the rear circuit. Not that it is right, but it seemed the M/C would only put pressure to the rear circuit when the pushrod is at the bottom of the stroke.

The rear circuit has a sliding piston, and at the bottom of its travel, physically connects to the pushrod. This is where I got that last bit if pressure when I bled the brakes without front circuit pressure. This is a feature so when the front circuit fails, there is still some pressure going to the rear circuit. But once we bled the front, that sliding piston wouldn't work correctly because it needs pressure from the front piston to move correctly. I think the shop needs to re-examine the seals on that sliding piston.

Sorry if this is getting confusing, but it's getting late, and I'm still trying to make sense of this!

Tom

[tyang]

It's fixed!!!

Sean at Bill Pollard's shop and I discussed what I learned from all the advice from the Forums and professional community and disassembled the M/C one more time. We carefully measured the suspect seal and found the ID to be slightly different from the one we had in a rebuild kit. This must have been just enough of a difference to cause our problem.

I've learned a ton about this particular M/C and the ability of certain rebuilders at the expense of a weekend, but I'm very happy it's fixed! Thanks everyone!

Tom