Hydroboost manual brake adapter plate

73 Mike

73 Mike

I'll drive it someday
Joined
Mar 30, 2008
Messages
714
Location
Boston, MA
Does anyone have a picture of the adapter plate for a hydroboost unit for a manual brake conversion? I know that they are thicker than those used for power brake units and I presume it will have provisions for two bolts through the firewall rather than four. I also presume that the placement of the pushrod will be higher on the firewall since the hole for a manual brake is a little higher.

I've never been very happy with how my brake pedal ended up after converting to hydroboost. My car was a manual brake car that had already been converted to power, though not very well. In particular, even adjusted all the way in on the shortest pushrod, I think the brake pedal is too high.

Dimensions for the different plates would be extremely helpful as well.

As always, thanks! :drink:
 
Is the pushrod for the current brake setup in the upper or lower hole on the pedal arm?
 
ALSO, on my '72, the length of the rod outta the HB unit has to be measured, cut and threaded for the stock fork to fit on it, and attach with the stock clevis pin to the brake pedal lever....I found that modding that lever was super critical, as I had already moved my pedal placement forward a goodly bit some years earlier, honestly wish I had cut that rod shorter...but too much effort to change it now...that will obviously affect the height of the pedal but there maybe variations on the HB unit used and so I can't say for sure...
 
There will be two holes in the brake pedal "arm". One will have the clevis mounted in it and the other will have the brake light switch bracket mounted in it. Power brakes will have the clevis mounted in the lower hole and manual in the upper hole.

BrakeBrackets.jpg
 
BBShark said:
There will be two holes in the brake pedal "arm". One will have the clevis mounted in it and the other will have the brake light switch bracket mounted in it. Power brakes will have the clevis mounted in the lower hole and manual in the upper hole.

BrakeBrackets.jpg
Click to expand...

What do you do with the switch then? Essentially change the hole used for the clevis rod for the one used for the light switch? If so, what happens to the plate thatthe light switch hits?
 
73 Mike said:
What do you do with the switch then? Essentially change the hole used for the clevis rod for the one used for the light switch? If so, what happens to the plate thatthe light switch hits?
Click to expand...

The manual brake switch striker plate is different than the power.
 
BBShark said:
73 Mike said:
What do you do with the switch then? Essentially change the hole used for the clevis rod for the one used for the light switch? If so, what happens to the plate thatthe light switch hits?
Click to expand...

The manual brake switch striker plate is different than the power.
Click to expand...

Isn't that interesting :idea:

Is there an easy way to tell the difference?

Edit : I wasn't being dense. I couldn't see the pic that you attached at work
 
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If you have manual breaks now your break light switch bracket will be in the lower hole in your break pedal and the clevis will be attached to the upper hole. For the Hydroboost to work properly you will need to reverse the two. My solution to this problem was to weld the break light bracket to the clevis. I had to drill out the hole in the bracket so it would not interfere with the pin going through the clevis. It was easier for me to do it like that, rather than order the longer bracket that mounts in the top hole. I'm sure some of the vendors sell the power break bracket, but I wouldn't want to install it after the time I had getting the pin out from up there.
 
Zips has a new stricker bracket. I think they are about $10. If your clevis is currently in the top hole you will need to move it down for Hydroboost.

To get back to your original post. If you think the pedal is too high, you can thread turn the rod of the hydroboost in the threaded clevis to adjust that.
 
BBShark said:
Zips has a new stricker bracket. I think they are about $10. If your clevis is currently in the top hole you will need to move it down for Hydroboost.

To get back to your original post. If you think the pedal is too high, you can thread turn the rod of the hydroboost in the threaded clevis to adjust that.
Click to expand...

Re tip 1, I'm going to spend some quality time under the dash this weekend to figure this out.

Re tip 2, the rod is already at its shortest. I even took off the jam nut to get that little bit shorter (3/16 maybe).
 
OK guys. You were both right. The clevis was in the top hole and the striker on the bottom. I'll order up the PB striker but in the mean time I'm going to put the clevis in the bottom hole and see if that solves the problem.

Thanks again for the direction. :yahoo:
 
You may not be able to move the clevis down enough. Your hyrdroboost unit is mounted currently mounted on the manual brake studs (that were used previously to mount your master cylinder). The mounting position of a power brake booster is 1.25 inches lower than the manual. This allows the booster unit to line up with the lower hole on the brake pedal arm.

It's not impossible. You probably could make an adapter plate as you suggest or you could re drill your brake pedal bracket.

Here is a picture of a power brake pedal assembly (on the left) and a manual brake pedal assembly (on the right). The two square holes in the manual ass'y are the stud holes for the master cylinder.

449d4b87774e00.jpg

These are the dimensions for a power brake booster mounting. The square holes in the manual bracket are 1.25 inches above the centerline of this.

449d56a26d82c9.jpg
 
I don't know what to think now

First of all, Shark I want to thank you for going above and beyond in helping me with this. I truly appreciate it.

After reading your post above, I've spent the last hour looking over all the components and I'm not sure what to think. Maybe I'll start at the beginning to help clear my mind.

This all started with poor brakes that I traced back to a bad master cylinder. Once I looked closely at the MC, I found the following that I had missed on inspection:

Shimstackonmastercylinder.jpg

On removing the stack of bolts and shims, I found the following stripped stud on the power brake booster:

Stippedboosterbolt.jpg

That certainly had to go as the stud isn't really replacable without huge effort. I removed the booster to find that it had already been replaced. There was a sticker from MBM Power brakes inside (not a GM affiliate I don't think). Also, I found that the firewall had been "modified." (TIM AT suggested that a Ginsu knife had been used for this mod):

prerepairbrakehole.jpg

From this, I drew the conclusion that the car was originally a manual brake car that had been shodily changed to power.

Based on this and the fact that I had to replace the booster anyways, I cleaned up the firewall and bought a hydroboost unit:

Shineyhydroboost.jpg
 
Now things start to get interesting. After installing and priming the hydroboost, I ended up with solid brakes (eventually) but after driving, It would begin to self apply until it would pretty much stop itself. This turned out to be a combination of the pedal not quite returning to the "rest" position and a fitting on the return line out of the hydroboost unit that had a very smallorifice that was causing a pressure build up. I fixed the fitting but never really got the pedal to return up to the rest position. Two patches worked ok. First, I'd just pull up on the pedal with my foot under the pedal. Not ideal. Second, a wrap of bungee cord to increase the return force worked ok. Again, not quite a permanent solution. That brings me to the start of this post. I had concluded that the car was originally a manual brake one that had been converted to power based on the firewall and the aftermarket booster.

Now I'm not so sure. The pedal assembly that I have absolutely coresponds to the one that you show on the left, the power brake one. Granted, the car is an auto so there is no clutch pedal, but there is no ridge on the side plates, nor the square hole for the stud.

What to think now? Perhaps more importantly, what is the next step?

Clearly, something has been added after the fact. Could be the pedal assembly. Could be the section of firewall.
 
It looks like your car was originally a PB car and someone "converted" it to manual. Here is a picture of the PB firewall cutout, that's what yours looks like originally:

PowerBrakeFirewall.jpg

If I were you, I would put the power brake setup back to the way it originally was. That sure seems like the easiest most effective route. I'm not a fan of hydroboost, except for trucks and tow vehicles and the vacuum assist setup is used very effectively on the vast majority of production cars.
 
Conclusions and next steps

After thinking about it further and looking at it further I have the following obervations and thoughts.

Regardless of how it left the factory, the car is now a power brake car. The pedal assembly certainly is and I don't intend t change it.

If you look closely at the mounting plate for the hydroboost unit (remember that it is mounted "upside down" to have the fittings point down) the barrel of the body is somewhere between the barrel of the power brake unit (dead center of teh four bolt holes) and near the top (dead center of the two square holes but below center of the four holes for the power brake plate). Conclusion: The ideal clevis mounting hole is probably somewhere between the top and the bottom hole.

Choices:

1) Revert to manaul brakes: Would require replacing the pedal assembly. I don't really even want to think about this job. I'd rather remove my own appendix.

2) Replace the hydroboost with a regular brake booster. Viable but probably not my first choice. May not solve the problem either because the hitch in the motion may be a function of the pedal asssembly rather that If I were to change the booster and the pedal still wouldn't return, I'd be back to removing and rebuilding the springs and bushings of the assembly. Back to choice 1 really as far as effort goes. Where's my scalple?

3) Switch the clevis rod to the lower hole and see what happens. I think this is the cheepest (no cost) easiest (a relative term to be sure) and most likely to succeed option. If this helps, I would still need to replace the striker plate and move/replace the mount for the brake light switch, but that wouldn't need to happen unless the problem of the return was actually solve.

4) Build a spacer plate. Kind of a strange choice. I don't think I should have to do this, but it may actually work. I think that a 1/4 inch thick aluminum plate would do the job.

The plan: I'm still waiting for engine parts. The disabled 400 small block is still in there. It's running but the rad is removed. In the mean time, there isn't much reason not to try switching the clevis to the lower hole. I won't know for sure if it works until I take the car for a run, but I think that I'll be able to see and feel if the hitch in the motion is gone.

Any other thoughts?
 
BBShark said:
It looks like your car was originally a PB car and someone "converted" it to manual. Here is a picture of the PB firewall cutout, that's what yours looks like originally:

PowerBrakeFirewall.jpg

If I were you, I would put the power brake setup back to the way it originally was. That sure seems like the easiest most effective route. I'm not a fan of hydroboost, except for trucks and tow vehicles and the vacuum assist setup is used very effectively on the vast majority of production cars.
Click to expand...

That's certainly an option. Part of the reason that I went to hydroboost is that the idle vacuum of the 400 was very poor. The stroker that I have going in may well be better (certainly designed to be) but I won't know until it runs.
 
To make things more complex, here are a couple more considerations.

First, the pedal lever ratio for a C3 is 5.4:1 for manual and 3.4:1 for power. So that means you will get 60% more pushrod force with the manual setup. That, coupled with the hydraulic assist that a hydroboost provides will make the brakes difficult to modulate. I think the StopTech website has a very good description of the forces in the brake system. Might be worth a look.

Incidentally, the pedal ratio for a C5 is exactly the same as a C3 and they have great brakes.

Second, At idle and low speed, you will not have much need for high levels of vacuum. You are not going that fast so stopping is easy. Also, the booster has a check valve that "stores" maximum vacuum levels as you are accelerating. It seems unlikely that a streetable engine would be unable to produce a good level of vacuum.

If still a concern, there are 12VDC vacuum pumps.

Lots of decisions to make!:confused:
 
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