275 GTB Brake Master Cylinder Question

Is there a security valve in the system?

 

I don't believe so.

I'm project managing this restoration, and the shop working on this issue and I are trying to see what is wrong. I suspect something was incorrectly installed in the secondary circuit, causing this strange condition of no pressure to the rear brakes. We have done every combination of gravity bleeding, vacuum bleeding, pressure bleeding, to make sure it is not something we're doing. The M/C has bench bled twice, and each time the fluid seems to flow freely between the circuits. It's when it's mounted on the car that things do not operate normally.

Thinking out loud so bear with me, we managed to get some fluid to come out of the rear circuit when the front circuit was empty, allowing for a lot of travel. The last 1/4 inch of travel pushed a little bit of fluid out of the rear brakes. With both rear calipers bled in this manner, we moved to the fronts. Front circuit bleeds normally, with fluid moving from the top of the stroke. Once all the air is bled from the front circuit, and the rotors show brake force, we moved back to the rears and we no longer get any fluid flowing, and no brake pressure to the rear calipers with the bleeders closed. Once there is good pedal from the front circuit, it seems there is not enough travel to push fluid out the rear circuit. I've adjusted the free play on both push rods, even over lengthening them to see if anything changes, but nothing.

After all this troubleshooting, the next idea is to remove the inlet port to the rear circuit while the M/C is still mounted on the car, and carefully watch the movement of the secondary piston. It should begin its stroke before the inlet port, and push past it when the brake pedal is depressed.

Before I scream bloody murder at the rebuilder, I want to make sure it's not something we're doing. Any thoughts are greatly appreciated!

Tom

 

First...a brake pressure gauge is a huge aid for troubleshooting this type of problem. Using one, the bench bleed can be confirmed...by ensuring that each circuit is really pushing fluid, as opposed to just moving it (I would have presumed that the specialist shop would have tested the M/C after their rebuild). But...you say that the M/C seems to bench bleed okay...

If that's the case, I suggest either an air leak in the rear circuit (split line, bad wheel cyl., poor junction/fitting, etc.)...or, a rubber hose deteriorating (of course, I'm presuming that no parts are missing...proportioning valves, or such)...

I restore motorcycles and have seen the deteriorating hose a few times. Essentially, the inside of a rubber hose starts to deteriorate and a sort of "flap" forms. The flap just floats when there is no pressure...allowing fluid to move under gravity. But, when pressure is applied to the pedal, the flap gets pushed up and blocks the circuit. The flap can form the other way, too...allowing the brakes to be applied, but preventing the flowback of fluid...resulting in a locked caliper or circuit.

Are you getting a "good pedal" when bleeding? That is, does everything seem fine with no rear brake power. Or, do you never get a good pedal?

If you get a pedal, I suspect the deteriorating hose. If not, more likely an air leak...

I'm not familiar with the rear brakes on the car...but, isolate each side and try bleeding. That is, if you replace the left rear system with a block-off plug (or, a brake pressure testing gauge), will the other three wheels bleed and work more correctly (the proportioning will be off, but it should reveal an airleak if one exists)? Same with the right rear?

I know this is obvious, but frustration can make one overlook the obvious: work methodically, progressively and analytically. Choose your method/approach...work around the system progressively...and, stop and think about how your observations fit into the big picture. It's just nuts and bolts (and seals and hoses)...a deep breath and some logic will surely find the problem.

Good luck!

 

Robert and Tom; The brake system you are dealing with is not overly complicated and it seems to me that you have done everything right in trying to bleed the system. I don't know if it's time to scream at the rebuilder, but you should send the cylinder back and let them disassemble it and give it another try. I'm betting that one of the seals on the rear circuit piston is on backwards - giving you some fluid output, but no sustainable pressure. Please don't be offended by this question, but are the rear bleeders on top of the calipers? I have had several instances over the years where people have brought in cars (not Ferraris) with 'unbleedable' brakes and found rear calipers mounted on the wrong side with the bleeders pointed downwards! If the Girling units you're dealing with were rebuilt and the piston bodies were swapped side to side on the caliper perhaps this could happen. Good Luck. Dave B.

 

A few years ago I had a similar problem getting proper function from a 275 master cylinder. The rebuild kit didn't exactly match the parts that had been in there before and if I remember correctly I ended up reusing one of the old seals to make it work right.

 

Tom,
I will be in NYC on Sunday-with the leaf spring retention jigs-which I told you and our Mutual Pal on the Hill I'd promised to fab for you guys...
As the both of you are familiar with whats behind My Green Door....the rahter simplistic reverse deductive reasoning is rather clear: ALL was good PRIOR to restoration-R&R, D&I, Fab/machine/BENCHTEST, and install...therefore, the lines, valves, booster, rubberlines(these GOTTA go, regardless!!!), and calipoers-if not split and re "pucked", potentially ignoring the internals and the bleed valve internals and orientation....this can ONLY lead to the item which was significantly altered from stock-the RE_SLEEVED Master...
Not at all casting aspertions on anothers work or a pox on his door, it is simple deductive reasoning....
I struggle with this issue constanly, using special diamond, blind hole hones for doing Masters, both alloy and ferrous....the real problem is obtaining appropriate seals and potential mismatching of additional other parts mixed and matched over the years(doinf battle with this same issue on another open 250 as we talk of your 275 problems...seal material and suitability with specific brake oils is aswell an issue, glad to yak on this on the week end-you have my cell number, give a holler and we can get specific.

In general, all fab guys will insist on "making it right"-I certainly am...what else in life is there except one's personal integrioty(your word)...
Being nearly certain of the identity of the practitioner, I'm certain he will gladly work this non functioning/performance failure/irregularity out with you until it's eventual conclusion.
These things are, in the final soulution, usually some obscure and minor technical fallability associated with the struggles on any 40-50 year old cars! And, usually easy to amend with minimal time effort and money.
CIAO, and good luck!
JC

 

Hi Guys,

Thanks for all your thoughts. Between the this forum and my forum, I'm sure we'll figure it out!

I'm beginning to agree with D.brum about the reversed seal. In my previous post, I found pressure to the rear circuit only at the last 1/4 inch of stroke, but when the front circuit was bled, I no longer got flow to the rears. Combining advice I'm getting from my Vintage Ferrari Forum, the internal sliding piston to the second circuit moves from pressure from the first circuit when everything is operating normally. I'm dying to get to work on Monday to look inside the M/C!

Just so everyone knows, EVERYTHING is new or rebuilt on this brake system, except for the M/C that was only recently sent out.

Hey, JC if you're on my coast, we'll have to get together. I'll call you.

Thanks everyone!

Tom

 

Hi Tom,

not pointing fingers here,... but a certain brake shop in Virginia rebuilt my MC. It still leaks..

My Servo was also sent to this establishment for rebuild.. when returned I installed it and it still would not work!


I sent the servo to Karps in California. According to them the servo was put back together wrong with certain parts backwards. They said the servo would have never worked...

Maybe its time to start over.

 

I have a similar story.
I use Karps exclusively. I'm tired of correcting "experts" poor work.
Morris

 

First off, I want to thank everyone who has lent their expert opinion, and support on this issue.

We had a discussion at the shop Monday morning after I distilled all the information I learned over the weekend, and removed the M/C one more time. We had a rebuild kit on hand, and decided to take one more look. It turns out the seal between the two circuits had a slightly larger I.D. than that of the rebuild kit. We replaced the suspect seal with the one in the rebuild kit, and it worked!

I've learned a lot about this particular M/C, and the shop doing the work has learned a valuable lesson about rebuilders! I'm happy because Bob is happy!

Tom

 

Occam's Razor.

 

Hi Jim,

If it wasn't for Occam, we could have been tearing the rest of the brake system apart!

Tom

 

Tom,

Thanks for posting the resolution to the problem here and on your own site. It helps a lot of us hobbists.

Regards,

Drew Altemarae

 

After local frustration with my 288GTO master brake cylinder which produced no pressure for the rear brakes, I sent it to Maranello for a rebuild and it has worked perfectly ever since. We thought it was everything but the cylinder as the local "expert rebuilder" was highly recommended. Wish I sent it to Maranello to begin with

 

The emphasis should be on replacing your fluids with regular intervals, so at least you don't get confronted with pitting in the cylinders, which would necessitate honing/re-sleeving.

Especially cars that are hardly used deserve attention in this regard; brake fluid is not a friendly substance, and brakes are generally a messy thing to work on IMO, a little spilled fluid in a concours engine bay can do a lot of damage.
In the end though, avoiding the presence of water in your brake system prevents a whole lot of problems, and a cylinder in good condition should be a pieve of pie to rebuild with the correct kit.