Hydroboost - Which MC?

My Bronco has finally been driven! Only to the end of the driveway and back, and with one somewhat major problem, but it went both ways under it's own power. The problem occured when I shifted from Neutral into Drive. The front end came up, the top side of the passenger coil spring squirted out of the bucket, the front end came down and didn't stop coming down until the fender met the tire. I had no intention of goosing it, but the shift kit must have made it jump into gear at just the wrong time and boom, bent fender.

Anyway, I have a question about brakes that I think I have answered but I'll ask anyway. I am running 4 wheel disk brakes with the hydroboost out of a 77 Lincoln Mark V. The master cylinder is from a 78 FSB. I have very little peddle and the throttle easily overcomes the brakes and moves the Bronco. I just found out that the master cylinder that came on the Lincoln has a 1.13" bore while the FSB master cylnder has a 1"bore. My question is, will the .13" difference be enough to cause my braking problem? I have good steering and I did the stop-to-stop bleeding process pretty thouroughly. I ran almost two of the large brake fluid bottles through the system to bleed it and had no bubbles so I'm confident that this is alright. My concern is in my brake peddle (if it doesn't turn out to be the MC). I did some fab work on my manual peddle assembly so the hydroboost system is lined up where the clutch used to be (more room around the engine seemed wise), the original clutch and brake legs have a plate welded across them both to provide a wider peddle, and the pivot point for the beddle is in a non-stock location because I didn't seem to have much throw at all with it stock.

I have a new hydroboost unit and a new MC on their way just in case either turns out to be the problem ($118 for the hydroboost and $17 for the MC at Advance Auto).

Sorry to hear about the spring popping out? No retainer I guess?
So is the rod mounted lower on the pedal arm than stock? If so how far?
Is your pedal bottoming out and you can't fight the tranny?
I'm running a 1" bore MC with 77 F150 front discs and rear drums with my hydroboost and I've got a fair amount of pedal travel. I would imagine with 4 wheel discs and a MC that small you may not be moving enough fluid to move the calipers to have full use of the braking, maybe only 80% of what you should. That said you should still have enough to win over the tranny but you might want to consider a larger bore MC.

MC bore size is independent of the booster and with the engine off it should feel as firm as manual brakes. Bleeding the booster has nothing to do with pedal travel to brake line pressure ratio. Since the booster effectively had a solid rod through it, it wont matter weither its assisting pedal travel or not. The same pedal position is equal to its corresponding brake line pressure weither the booster is working or not.

Kinda of a stupid question and don't take offense but do you have the calipers on correctly and not upside down/wrong side. I fought brakes for way to long and finally realized my mistake, it acted just like what you are describing.

Ben

Thanks.

In the world of MCs, does 1/8th of an inch make as much difference as it seems to? Any idea how far the hydro-rod should stroke? Thanks for the tid bit on how the hydro works, I wondered if it was a solid rod giong through.

I'll have to look in the morning to see where I moved the pivot point on the peddle. I did that a year ago and haven't looked back. Odd how I just got to the belt system but that's what 3 kids and a wife'll do.

You're right, my peddle hits the floor with seemingly no effort and that's why I can't stop (keeping in mind that I've never had it to the street yet) very well.

Regarding the springs, you're right, no top bracket was/is installed. It will be before the end of the weekend though.

bigmuddy said:

Kinda of a stupid question and don't take offense but do you have the calipers on correctly and not upside down/wrong side. I fought brakes for way to long and finally realized my mistake, it acted just like what you are describing.

Ben

Click to expand...

Ben,

No offense would be taken, I asked the same question of a guy who knows a million times more about Broncos than I ever will and he did have his upside down. Mine are correct though. Thanks,

A 1" bore has an area of .785 sq in.
A 1-1/8" bore has an area of .994 sq in.
A difference of about 21%
So the pedal throw on a MC with a bore of 1-1/8 would be 21% less than on a 1" bore MC to achieve the same line pressure.
Lowering the attachment point on the pedal arm would give you slightly less mechanical advantage and result in a shorter required pedal stroke to fully compress the MC.
By chance did you remember to bleed the MC by cracking the line fittings that plug into it? I was lucky to only go through a half a can of fluid before I realized this.

I didn't know that I should bleed the MC. I just assumed that I'd be fine bleeding it through the lines. The entire braking system is brand new except for the hydroboost so the lines were empty when I started. That was with the other MC though so I'll try bleeding the MC just to see if there is any residual air in the ports.

Thanks. I understand the physics/mechanics involved but thanks for the info. I moved the peddle assembly straight back, toward the driver, if I remember right. I was at a picnic most of the day and not under the Bronco where I belonged. I'll report on that later tonight if things work out the way I hope they do.

Thanks again,

OH BOY, I cant keep my mouth shut any longer so - - -

Bench bleed the master, I thought the same way you did and learned the hard way it don't work. one this note whats wrong with the lincoln master? you got the brakes from it why not the master?? this would work perfectly for you.

As to the pedal pressure a couple things come to mind.
check the intermediate rod between the hboost and the master. little or no slack here. Check the rod between the back of the hboost and the pedal, 25 thou slack no more. sounds like you have a good hboost install as your steering is not acting up.

write if you have problems or questions
TOFIC

I've installed the Lincoln MC and I do have peddle now, but it still isn't where normal brakes would be.

TOFIC, I was waiting to hear from you on this, thanks. Instead of removing the MC, I replaced the proportioning valve lines with lines that looped back into the resevoir. I then pumped the brakes (slowly so the fluid wouldn't fountain onto the floor) for quite a while and until I saw no signs of bubbles coming from either the ports in the bottom of the resevoir, or from the lines. I then hooked the prop valve lines back up. Is this adequate?

Regarding the rods, I have only a slight amount of play in the peddle/hydro unit rod, but it seems to push quite a ways before even thinking about engaging the brakes. I will be going out to check the hydro/MC rod shortly.

I have a helper coming over this afternoon (my dad) so I'll have his fresh eyes to look things over. He has a way of finding stupid oversites on my projects so we'll see.

I found this link rather useful on the operation of hydroboost including bleeding.

http://www.babcox.com/editorial/bf/bf030542.htm

Joe

jAMJET, Great article, thanks.

drop me a PM got several questions and no use plugging the forum.
TOFIC

I finally have brakes!

I had to modify my brake pedal assembly so it didn't bottom out so soon. I had just under .80" of MC stroke to start with, now I have 1.50" thanks to the removal of a small wedge of material and a nice weld (my pedal assy actually has two arms, one from the old clutch and one from the original brake). I'm guessing here, but I think the correct stroke is the 1.30". If you're bench bleeding the MC, there is a hesitation at that depth before it continues to almost 1.80". One noteworthy thing is that the MC pushes fluid from both reservoirs for the first 1.30" but only out of the rear reservoir (for the front brakes) for the final .50". I'm thinking that this is to accomodate the more likely blow-out of a rear brake line (rust along the frame) by giving the fronts just a little more oompf. Thoughts?

You thought makes sence from a saftey standpoint, also if you are ever braking hard enough to need that much pedal travel, there is so much weight transfer to the front wheels that the rear wheels are likley to have already locked up and more pressure wouldn't help.