Should I be able to lock up my tires? (Led to Leed vac pump install)

[Dakota]

On episodes of "Fathom Works" that I've watched on TV in the past, their repair/restoration work is often road tested by driving the car near the shop, stomping on the brakes and then checking the skid pattern from all 4 tires.   That memory got me wondering now that I've I finally finished replacing a leaking master cylinder (AutoZone purchase if I recall correctly) with one purchased from Dr Diff:   Should I be able to lock up all 4 of my tires?

My 70 Challenger has power brakes with a 1-1/16" diameter master cylinder.   It originally had drums on all 4 corners.  I changed the front brakes to a single piston caliper with 11" rotors using a Right Stuff Detailing kit.   The tires are Nitto 555 G2, 255/45 ZR17 in the front and 245/55 ZR18 in the back.   The rear drums have 15/16" diameter brake cylinders. The plumbing for the brakes is otherwise all stock.   The booster is original to the car.   I have a 340 and a cam of unknown specs.   Based on the data logger in my FiTech, engine vacuum is running between -13 and -14 inches of Hg at idle.

Now that my brake pedal isn't bottoming out, the brakes are noticeably better.   That said, I've hit the pedal pretty hard a couple of times, but no skids so far.   I don't have any experience with this car before I took it apart or any other classic Mopars to know if this is normal or not for the way my car is currently equipped.     I've followed along through CudaJason's journey with hydroboost (I have power steering too) and also just read HP_Cuda's thread about adding an electric vacuum pump.... all good upgrade options to consider if what I'm seeing today is as good as it's going to get.   The next step up to bigger discs and multi-piston calipers is also available but pretty pricey.

Thanks in advance for your feedback and guidance.

[dodj]

Yes you should. Stock kind of set-up. The rears should lock up. Maybe not the fronts......

Edit..what I meant by that was the rears would lock up first and the fronts would not lock up at the same time. My experience was you would be loosing the rear due to locked up tires and you would be releasing brake pressure to regain control. So I added an adjustable valve to reduce rear brake pressure.

[Racer57]

I'd work on bleeding the brakes again. Start with the easy stuff first before you start replacing parts.

[Chryco Psycho]

Yes you should be able to lock them up .
Solving the issue can be difficult , I really like the hydroboost conversion if you are OK with far from stock .

[70vert]

I agree, @Chryco Psycho
I tried everything on my 70 Challenger w/440 and was never satisfied with braking performance. Definitely could not "lockup" the tires. I finally had a Hydro-Boost system installed by a local shop with experience doing them and WOW, the difference was amazing. Obviously it is not for OEM Show cars but if it's a driver I would seriously consider it. In addition to stopping, it cleans up the area somewhat getting rid of hose from carb and the big vacuum can on the firewall. I actually like the looks.

[MoparCarGuy]

Maximum braking occurs as the driver modulates the pedal to reach the front braking limit just prior to front lockup with no rear lockup. The goal is to get maximum brake line bias to the front tires without lockup since the majority of braking is accomplished by the front tires. With enough driver input, all four tires should be able to lockup on our E-Bodies. Front locks first, rear locks second.
You want the front tires to always lock up before the rear tires. Think big rig. If the rear tires lock up first, the car will spin as the rear will slide around like a jack-knifing tractor-trailer. The rear tires should only lock up after the front tires lock to prevent the car from spinning.

[Dakota]

It turns out that Leed Brakes' headquarters are local to me.   I stopped in to talk to one of their reps.   They confirmed what I've been reading here and elsewhere online that engine vacuum on warm idle needs to a minimum of  -17 inches of mercury (Hg) or higher (more negative), with something in the -20 range being preferred for a vacuum power brake booster.  I'm in the -13 to -14 range, so that's what I believe I need to improve.

I had capped the ported and non-ported vacuum taps on the FiTech throttle body to see if the vacuum improved.  It didn't.

I ended up purchasing one of the Leed's Bandit vacuum pumps.  Finding space for it may be a challenge as I have wire junctions occupying the space under the battery try.  If I can't find any place better, it'll probably end up in the trunk.  Any vacuum line longer than the 6' they provide has to be hard-piped (3/8") and I need to use a heavier gauge wire (16 -> 14) to reduce voltage drop for the longer wire run.   We shall see.

I'll tack some info onto HP_Cuda's recent thread if there's anything worth documenting.

[MoparCarGuy]

I also have the Leed VP002 pump. It is mounted under the battery tray. You might consider inside the grille area or the driver-side fenderwell. Consider fabbing a splash shield though.

[Brads70]

What about the age old Vacuum Canister/Reservoir?  Pretty common on Fords and some Pontiac's back in the day? AKA the old tomato juice can , https://www.summitracing.com/parts/opg-g241078
Cheap enough to try?

https://www.summitracing.com/parts/mrg-3701

[70vert]

but doesn't that just "hold" more vacuum, not "create" more vacuum? so you'd still need a pump to fill it up if you are not getting enough vacuum from the carb.

When trying to solve mine I did try a vacuum pump but was still not happy with it. It was not a Leeds, so maybe there's is much better. I may have missed the specs, how much vacuum does the Leed Bandit model generating?

[MoparCarGuy]

The LEED VP002 can achieve a little more than 24 in. Hg vacuum at sea level.

[Dakota]

You might consider inside the grille area or the driver-side fenderwell. Consider fabbing a splash shield though.

I used the chat feature on the Leed Brakes website to connect with their tech support to ask about mounting the pump in the wheel well (nice folks, by the way).  The main concern the tech expressed would be getting the relay that's inside the case wet.  After opening the pump case, I think it will be relatively straightforward to relocate just the relay to the area underneath the battery tray and then mount everything else in the wheel well.  This should better protect the relay from water.  There's one other potential water entry point on the pressure switch near the hose connection port (see pic with blue wire).   A little shot of silicone grease should take care of that.  We shall see!

[Dakota]

Finally got the vacuum pump installed and running.  WOW what a difference!   I was expecting a stronger response from the booster but it was even better than I thought it would be.

I ended up making a bunch of small modifications to get the pump and case into the driver side wheel well.   The backside of the core support (roughly behind the high beam bulb) would've been the easiest place to put it, but I already have a relay box mounted on the front side of the core support in that area, so I ended up mounting the pump on the side panel of that same part of the core support.  Hopefully the pictures below will help make a little more sense of the description.  This location involved a bit of jockeying to work around the side marker light harness and the "dimple" in the core support behind the dual beam bulb.  Some clearance work on the case was needed to get it to fit next to the dimple.   That's my wife's red lipstick on the dimple which helped me figure out how much of the case to remove... just in case you were wondering. 

I made a mounting bracket out of some scrap metal.   Like me, it's not pretty but it worked.  Sorry about the blurry pic. 

The relay from inside the vacuum pump case was relocated to a spot under the battery to keep it from getting wet.  This involved splicing in about 2 feet of wire into the relay wiring harness and then adding a hole with a rubber grommet to the case so the thicker wire bundle (4 wires instead of just 2) could be routed out of the case. 

I didn't take a picture of it, but the vacuum hose provided by Leed was routed under the core support, then between the battery and wheel well, under the master cylinder and then to the booster.  This took all 6 feet of the process's hose (anything longer would need to involve hard pipe).

The vacuum hose ID seems to be about 1/16" larger than the nipple on the brake booster, so the stock spring clamp was replaced with a screw clamp to make sure the hose was secure.

The pump positive lead comes with a 15 amp inline fuse.  The line was routed to the "ACC" location on the fuse panel. 

With the key "on", the vac pump comes on and, in my case, the electric fuel pump also cycles on for about 5 seconds.  That's a pretty good hit on the battery, so we'll have to see how that works in the long run.

It wasn't a terrible upgrade project, other than it seems like gravity is holding me tighter to the garage floor when I lay on my back.  It's been just one day of driving, but so far I'm very happy with it.

[71-440]

That's a nice setup. When I first got my car I had to stand on the break pedal to get it to stop. I was thinking about the Hydro-Boost after talking to Neil.
Unfortunately I ran into a different problem that needs fixing first.


Joe

[Solarguy]

So............Can you lock em up now??