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Discussion Starter · #1 ·
On my 77 GTB it appears that m

On my 77 GTB it appears that my rear brakes are working better then the fronts. I had the bias valve rebuilt by whitepost a few years ago so I don't suspect it to be bad. Does anyone know what the bias pressures are (F/R). I'd like to put a gage on this to see if it is in spec. I have a set of braided hoses I didn't put on yet but I'll wait to see if the pressures are OK.
 
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Discussion Starter · #2 ·
Steve,

I'm a bit confus


Steve,

I'm a bit confused, in that I didn't think that the 308 used a brake bias valve. Perhaps you referring to the brake pressure differential valve that was rebuilt? This is the valve that differentiates front and rear brake pressures, and trips a warning light in the event of a major pressure differential.

If the rear brakes are performing better that the front, I would suspect that the front rubber brake lines are restricted internally, which does happen over time.

How does the brake pedal feel?

Regards,
David
 
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Discussion Starter · #3 ·
Hi Steve,

David is correct,


Hi Steve,

David is correct, no bias valve.

The brake bias is controlled by the pistons/stroke in the Master Cylinder.

You don't say much about past work, service, upgrades, and there are ALOT of things that can cause Brake Performance issues on a 26 yr old car!

Everything from air in the lines to glazed pads, to atrophied brake hoses and more can cause poor brake performance issues..

How about some background info detailing more of the specifics of what's been done so far..

Regards, JRV
 
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Discussion Starter · #4 ·
OK I was under the impression

OK I was under the impression that the pressure diff. valve was a bias valve which gave you a higher brake pressure on the front vs. the back. And with this design that pressure was fixed. But back to my status here. The car has the orig. brake lines and has had the normal yearly brake fluid change.Pads were replaced once and the car has 45k on it. Pads still look good but you can tell that the front rotors are not getting the same brake pressure as the rears. Also when you nail the brake peddle you don't "lock up" the front brakes. So I think I either have sized cal's on the front or the hoses are shot. Peddle feels good. I'll put the car up on the 1st snow here and will rebuild the fronts and put on the braided hoses.
 
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Discussion Starter · #5 ·
Glased pads or rotors. Simple

Glased pads or rotors. Simple bias test,, car on jack stands, push down slowly on brake petal until another person can feel lockup on front wheel by turning it until it is very hard to turn. Hold brake petal at this point and have helper check rear wheel. it should turn a little easier than the front. Used this trick to help setup the starting point for formula race cars before data aqu was availible.
 
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Discussion Starter · #6 ·
>>Glased pads or rotor

>>Glased pads or rotors<<

that is my hunch also.

common on vintage 308's/Dino 308's.

JRV
 
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Discussion Starter · #7 ·
Some of the 308s do actually h

Some of the 308s do actually have a bias valve. You can see it (strangely labelled !!) in the parts diagrams on the owners.ferrari site. Part no. 95692019. I replaced a bias valve on my GT4 because it was leaking - it's a simple affair, one pipe in, one pipe out, in the front-rear brake pipe. According to owners.ferrari it's fitted up to 1977 on USA spec cars. Mind you, even if the valve fails in some unusual way, it can only change the pressure going to the rear brakes and, at most, it can only equal the pressure going to the front brakes...... The probability of the problem being due to glazed pads / discs is more likely. >>>>>>you can tell that the front rotors are not getting the same brake pressure as the rears>>>>>> How can you tell?
 
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Discussion Starter · #8 ·
Well...

Actually no. It&#39


Well...

Actually no. It's not a bias valve, but rather a "saftey braker" type device which dampens the hydraulic pulse to the rear brakes, in the event of a panic type stop...hence preventing premature rear brake lock-up.

Regards,
David
 
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Discussion Starter · #9 ·
Yes, my error, "pressure l

Yes, my error, "pressure limiting valve" would be a more accurate description - but almost anything is closer than how it's labelled on the owners.ferrari site.......
 
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Discussion Starter · #10 ·
Ahhhh...Yup!

Words sure do


Ahhhh...Yup!

Words sure do get a bit lost in the translation, and it does seem that the same "theme" occured when the Owner's Manuals were written, as well.

Regards,
David
 
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Discussion Starter · #11 ·
If your front rotors look like

If your front rotors look like they have a shiney glass finish on them instead of a cast iron look and feel, they are glazed. Same thing for the pads. Old trick that works just fine, sand the rotors with 80 grit paper until you get down to the metal.Sme thing for the pads, sand the glaze off. Next brake job--- be sure to bed the pads slowly to allow the pads to "gas out" before you really jump on them.That is what caused the glazing problem. Or use a prebedded pad like Performance Friction Carbon Metallic.If your petal is hard as stated, and you did the quick bias test, glazing is the problem.
 
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Discussion Starter · #12 ·
bob,

welcome to Ferrari-Tal


bob,

welcome to Ferrari-Talk and thanks for the Tips.

To Re-iterate Bobs advice, whenever installing or breaking in new pads (even on new cars)it is very important to "bed in and cure" the pads and rotors correctly to insure proper performance, avoid glazing and minimize the possibility of squeking.

I'll see if I can find a link to a good Brake Pad Break In Procedure article and post it here.

Regards, JRV
 
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