When manually bleeding the brakes by pumping the pedal on a dual-reservoir MC system, why does the pedal go all the way to the floor? I thought the safety feature of a dual-reservoir MC is that if you rupture the rear brake line, e.g., you would have some front braking. But isn't cracking open a bleeder valve in effect rupturing the line?
The first time around it will tend to have little to no pedal. If you have drums, make sure the adjustment is up and bench bleed the master first off the car. For general bleeding, if the shoe adjustment is good, you really should not be hitting the floor with the pdeal.
When you lose one half of a dual master cyl, the car will be much more difficult to stop, and the pedal will sink way down compared to normal operation. I will always bench bleed the cylinder, then leave the lines cracked slightly at the master, & back bleed the system a little before I tighten them. This speeds things up a lot, since the air goes right up (where it wants to go) instead of down through a maze (where you must force it to go.) Don't back-bleed so much you pollute the new master cyl. Just get the bubbles out & tighten the lines, then bleed as normal.
If you get it all bled, and then open one bleeder, it should not go to the floor. If it does, you might as well just replace the dual cylinder with a single. Pedal ratio/travel might be the issue, a dual cylinder needs more travel than old pedals provide.
There have been previous debates on this issue on the HAMB, might try a search if you are interested in the topic.
Interesting. I'm running single master cylinders on 3 of my 4 old vehicles. I guess I'm not missing much.
This dual master cylinder is a common complaint. I have read a lot of threads on even new cars totally or almost totally loosing the brakes with a brake line failure. You would think you would have at least 1/2 the braking ability, but according to accounts more like 20% to none.
It would depend on which "end" of the car you lost the brakes on. Front brakes (depending on the car and the brake design it has) provide roughly 70-80% of the stopping power. - EM
In my 92 OT F150..Lose a rear line and pedal goes to about 1" from floor but firm from there..Scary but you stop..Lose a front line and pedal goes to floor with some rear brake but not firm and more scary...Have a Wilwood dual in another car with drum/drum and pedal will go way [way, way] beyond half way if you bleed one end or the other..?..
Put 2 masters and a balance bar on there and you'll know that you will have brakes if one system fails.
That is quite true, when both are working. You might find the reading interesting on the Chevy (newer cars) guys that are loosing brake lines in the northeast. They almost all claim no brakes. This has resulted in a fair number of collisions and fatalities (if you believe the stories) and talk of class-action law suites. I have no idea about the validity of all this, but reading the chatter is interesting.
Yes and No - or should I say maybe! With a balance bar you must check the pivot travel to ensure the pivot will "bottom" out before the pedal does - once you know THAT - Then you will be sure one system will work if the other fails. I have my coupe set up this was and had to make the balance bar twice to get it right! ;-)
Is it a new master cylinder? I have heard more accounts of shit master cylinders out of the box lately than I can remember.
Jseery, I too lost a brake line on a late model car ("99 F150) and the pedal went to the floor and I also have been wondering the same thing as the OP. If the dia of the master cyl. can fill the calipers and apply the brakes, is it not the volume of fluid going to the front if the rear has a leak?
Not sure exactly what to call it or GMs technical name or all of the applications you can find it on, But i know its on 80s Chevy trucks. The centering valve that controls the brake warning light is a pretty neat contraption. Should you loose a line, the lack of pressure on one side lets the line pressuer move the plunger valve over to close the open side. It's not 100% seal but applying blake pressure increases the seal. it goes a long way toward keeping pressure and getting you stopped.
Let me try re-phrasing the original question; While manually bleeding the brakes by pumping the pedal on a dual-reservoir MC system with proper pedal ratio/travel, should the pedal go all the way to the floor when a bleeder valve is cracked open?
This topic hits home for me- I was towing a boat with my '04 Yukon when my back brake line blew. The salt brine they use here in the northeast rusted it out pretty badly. With a 6000lb boat behind me I got it stopped. It went ding ding ding and the Service Brake System message came up on the dash- I had just enough pedal to get it stopped once, then to the floor the pedal went and barely any brakes at all. While not perfect I will take a dual M.C. over a single any day of the week.
The parts have to match. If you don't have the correct bore, you are using too much stroke to actuate the pistons in the disc. When I swapped the jaguar calipers in place of g-body calipers, I had to get a firewall mount 1 1/8 with booster instead of 1" dual chamber manual master just to get pedal back.. When I put the disc brakes on a decade ago, I had to lose the 7/8 master for that same 1" for the same reason. the drum/drum got by with not much fluid movement.
For an initial bleed after a master change or installing a new system, the pedal can and probably will go to the floor until some of the air is removed. For those who bleed manually, whoever is working the pedal, they should know not to hit the stop too hard. And again, the adjustment on the rear brakes (if there's discs up front) will have a significant impact on how far the pedal drops.
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