Brake feel question

These are old cars. What are the chances the brake booster or master cylinder have been removed and rebuilt improperly or improperly sourced? Just because it fits does not mean it will do the job at hand. Any change in master cylinder volumes will not match with the engineered caliper piston sizes. Result long pedal. Also have you pulled the master to see if the booster has an adjustable pin that goes into the back of the master? The pin needs to go into the back of the master to its stop and then have about 20 thou of play. There could be a WSM spec perhaps? booster pin too short and you get a long pedal.

Oh and I'm just spitballing. I do not know anything about boxers

 

Then it is probably something else. An internally leaking master cylinder can also give you a long pedal. Seals and bore fouling can easily happen in an almost 35 year old car. Also old rubber brake lines that are internally fouled or breaking down can give very weird symptoms. If those are original I would change those just because of age. There are simple tests you can do to a vac booster to verify its action but it sounds like it is working.

 

Wade Williams here. I am the lead tech at *****************. In my experience, in my experience, if you have a firm petal with no vacuum, it is a bad booster.
I test them like this: pump the brakes until all vacuum is consumed, the petal should be very firm. Stat the engine, the petal should move down slightly. If it goes way down, it is a bad booster.

 

Wade, Just for knowledge, why does the pedal travel more when bad? If it was a leak it would have no effect? Or is the diaphragm stiff and not working?

 

I really don't know the inner workings of the booster. I have had this issue many times and it always goes that way. I guess I should be more proactive and figure out why it does that. I haven't had a booster apart to see how it works. Maybe some engineering people on here can chime in with the how and why.

 

Don, About an inch sounds normal. The booster always causes some movement. If you were close I would say just stop by and I'll try it and see if I thought it was odd or too much. If you happen to drive through the Atlanta area, look me up.

 

After all the other tests and diagnosis you have done, it is not a stretch to think this sounds more like a problem of brake bleeding. I may be incorrect, but is not the clutch slave cylinder tied into the brake hydraulics system, so make sure the clutch hose is new and the slave is not leaking. At the least bleed this line at the top of the slave for greater certainty. The slave's seals and hose are subject to alot of heat, the hose might be internally degraded, the seals may be compromised. These are often overlooked until the seal or line fails outright, I view them as proactive replacement items.

Many would use the Motive pressure bleeder or a vacuum bleeder, but sometimes just topping up the brake master and letting each caliper bleed screw loose one at a time, never touch the brake pedal and let each one drain/drip slowly overnight by gravity only can solve for a difficult bleeding problem. When the whole system is worked on and goes dry, lots of opportunity for an air bubble/pocket to remain no matter how bled. Sometimes repeated bleeding is necessary. The gravity method ofetn works where other methods don't, the only downside is it takes waiting time to do.

Also, the type of pad used can make a huge difference in brake feel. You might have always been using "racing" pads or some other high performance formulation, that is not actually well suited to normal street driving. The OEM ATE brake pad composition is the way to go, but in any event a street pad is best for the street. High performance pads can feel very wooden and have low initial bite at non race temperatures. We all think high performance means better, for brake pads it is more a case of different composition for different purposes. Ask what specific type of pad was put on. And one needs to make sure the discs are skimmed and have no glaze embedded in them, which can notably reduce initial bite and affect pedal feel.

 

One way to see if it is a bleeding problem is to try a pump up test. After letting the car sit over night, this lets any air find its way to the same spot. It also lets the air relieve itself of any pressure it may be under. Start the car, if you have to have the brake pushed to start the car, only push it enough to make the car start. after it is running push the petal firmly, hold it, then release and push again. if it has air, the petal will be long the first push and become less long as you continue to release and push again. In this test the air pocket takes up pressure in the first push and as the air is compressed on the next push, the petal becomes more firm. Many times I have seen the fluid have micro bubbles that will allow a firm petal until the car sits and the micro bubble come together as a big air pocket the can be seen when you bleed it. This happens when you have had everything apart and no fluid in the system at all. Again, I am not an engineer, this is just my experience over the 22+ years I have been doing this.
This also applies to clutch bleeding.

 

You could have to much play in the brake booster push rod. This can be adjusted - marked "A" on drawing. I've done this on cars before to get a more instant brake feel.

I have the same problem as You on my 400GT and have taken the same route. Overhauled calipers and brake booster but to no avail. At least now all is fresh and new but still same brake feel. I will be changing the master cylinder this season to not leave out any component - then I can also check the push rod play.

If your master cylinder was leaking then the pedal would slowly bottom out if pressed continuously.


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To check If your master cyl. is leaking internally: You push with very little pedal pressure. Then it will leak past internal seals. If you push hard sometimes it will not leak internally.

 

Be very careful when adjusting the booster push rod, too tight and you have difficulty bleeding and brake drag will result.
As little as a 1/4 turn can be the difference in working and brake drag.

 

Yes,I've learned that the hard way. In most cases the push rod adjustment is just fine and should be left alone.

 

Is there a bleed valve on the master cylinder on the BBi? Could not tell from the parts catalog or OM. Air in the master cylinder is often the cause of a soft pedal on modern Ferraris. OM says the free play in the pedal should be 0.32-0.40" (8-10 mm).

 

Ah yes, separate system on the boxer, shared system on the later TR, Mondial, etc.

 

No, Boxers do not have master cylinder bleeders. I have in the past used absorbent mat with plastic underneath, in case anything gets through the mat.
I then crack the line at the master cylinder and bleed it there. It takes very little fluid to bleed it there. I also use a pressure bleeder to ensure I am moving the fluid and not ingesting air.

 

Shared on the TR, but the clutch reservoir pick up is higher than brakes. So you don't starve the brakes.

 

After bleeding the M/C, flush the area with clean water to wash away excess brake bluid.

 

It has been mentioned once before.....+30 years old flexible rubber brake lines are prone to appear soft and will often expand when exposed to internal pressure. It goes without saying that these should be changed out prior further fault finding on the system. Other than that it sounds like good sense to check the brake piston plunger tolerance and at the same time verify correct brake cylinder diameter. Anyway i would start with fresh rubber brake lines.

Best regards

Peter