A Question about Brake Pressures / Flow

[JamesDean]

Morning everyone,

I installed a new master cylinder on my 190E last night. I bought a nice Napa master cylinder bleeding kit and while I was bleeding it I noticed that the side port (closest to the rear of the car) flows fluid at a earlier point in the pedal travel and the top port.

One had to go more than 1/2 pedal travel or so to get much fluid flow at all from the top port, while the side port flow nicely through all pedal travel.

This new master is properly paired with the booster. Both are E420/E500 units so I know the push rod inside the booster is the correct length.

My questions

Is that normal?

What calipers are fed by the side port? and top port?

What happens if I reverse the ports (assuming it were easily done)?

For years now I've been trying to figure out why my brakes take so much pedal travel to engage and grab well. I've been through many master cylinders and three different boosters, trying various things. Nothing has made a substantial difference.

This pass I am working my way top down making sure everything is as it would be if the car was a E420/E500. So as of right now I've got:

E420/E500 Booster (used)
E420/E500 Master (new)
E420 Front Calipers (rebuilt from NuGeon - 2/3 years old)
SL600 Rear Calipers (rebuilt from Centric - 1 year old)
Textar Pads
R1 Concepts Rotors (1 year old)

[Elektri]

I'm not familiar with the brake system in your specific car, but for cars in general...

Brake pressure / power is not equal to all wheels. Rather 3/4 of the braking pressure goes to the front wheels and 1/4 to the rear wheels typically.

Then if you have an ABS system, those have electric valves and chambers which must be bled in a certain manner or there will be air trapped in those areas. That can cause excessive pedal travel. Consult a factory service manual for proper bleeding procedure of an ABS system.

[pmckechnie]

Which car?

[97 SL320]

The staggered flow is normal _during bleeding only_ .

A single circuit master cylinder ( or clutch master cylinder ) has a piston directly actuated by the push rod.

A dual circuit master cylinder ( tandem master cylinder used since the 60's ) has a floating secondary piston ahead of the actuated primary piston. When the system is full of fluid, there is a column of fluid between the two pistons. As you push the primary piston fluid comes out of the rear port and also pushes the floating secondary piston. Fluid ahead of the floating piston then exits the front port.

If you have a leak in one circuit, that piston must bottom out until the good side can make pressure.

Dual circuit masters split the brakes front to rear on rear drive cars , diagonal on front drive cars and triangular on older Volvos where 1/2 of each front caliper and one rear are on one circuit.

Generally swapping lines on ports isn't done. In a disc / drum system the drum side circuit has a residual pressure valve ( 2 to 6 PSI ) that keeps a bit of pressure in the system, this keeps the wheel cylinder cups expanded to prevent leakage. ( Sometimes on a race car a RPV helps reduce peddle travel on a disc system. ) Brake balance will be off with a stepped bore MC if you swap.

Brake peddle ratio ( ultimate pressure ) can be influenced by master cylinder diameter. Assuming the same foot peddle pressure. A small bore will make higher pressure but need more travel to make the same volume. A large bore will have lower pressure, higher volume and need less travel.

Some master cylinders have a stepped primary / secondary bore ( 2 diameters ) This can be used to design in front to rear brake balance.

Other times the primary piston is further split into two bore diameters, the large diameter provides large volume to push the front caliper pistons to the rotors. As pressure rises, fluid is bypassed from this area and the smaller area provides needed pressure ( leverage ).

Have a look at general fluid power ( hydraulic / pneumatic ) theory for how a hydraulic system works.

If you install calipers with larger pistons and don't upsize the MC bore, you will have long peddle travel.

[JamesDean]

That's very informative. Thanks!

It occurred to me as I read your first line that "loading" (rather the effect of having something resist the fluid pressure--IOW the caliper) might have a substantial effect on operations.

I do believe my calipers and master cylinder are appropriately sized, or at least Merecedes determined this as they are from the same car.