normal_post in Should I be able to lock up my tires? in Wheels, Tires, Brakes, Suspension & Steering Author Topic: Should I be able to lock up my tires?  (Read 511 times)

Offline Dakota

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Should I be able to lock up my tires?
« on: September 20, 2022, 06:03:59 PM »
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.



Linkback: https://forum.e-bodies.org/wheels-tires-brakes-suspension-and-steering/12/should-i-be-able-to-lock-up-my-tires/26475/

Offline dodj

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Re: Should I be able to lock up my tires?
« Reply #1 on: September 20, 2022, 06:12:10 PM »
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.
« Last Edit: September 21, 2022, 03:52:26 AM by dodj »
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Offline Racer57

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Re: Should I be able to lock up my tires?
« Reply #2 on: September 20, 2022, 07:17:36 PM »
I'd work on bleeding the brakes again. Start with the easy stuff first before you start replacing parts.

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Re: Should I be able to lock up my tires?
« Reply #2 on: September 20, 2022, 07:17:36 PM »

Offline Chryco Psycho

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Re: Should I be able to lock up my tires?
« Reply #3 on: September 20, 2022, 08:34:28 PM »
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 .
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Offline 70vert

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Re: Should I be able to lock up my tires?
« Reply #4 on: September 21, 2022, 10:35:02 AM »
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.


Offline MoparCarGuy

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Re: Should I be able to lock up my tires?
« Reply #5 on: September 22, 2022, 05:55:53 PM »
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.


Offline Dakota

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Re: Should I be able to lock up my tires?
« Reply #6 on: September 23, 2022, 07:38:13 AM »
It turns out that Leed Brakesí headquarters are local to me.   I stopped in to talk to one of their reps.   They confirmed what Iíve 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.  Iím in the -13 to -14 range, so thatís 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 didnít.

I ended up purchasing one of the Leedís 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 canít find any place better, itíll probably end up in the trunk.  Any vacuum line longer than the 6í they provide has to be hard-piped (3/8Ē) and I need to use a heavier gauge wire (16 -> 14) to reduce voltage drop for the longer wire run.   We shall see.

Iíll tack some info onto HP_Cudaís recent thread if thereís anything worth documenting.

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Re: Should I be able to lock up my tires?
« Reply #6 on: September 23, 2022, 07:38:13 AM »

Offline MoparCarGuy

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Re: Should I be able to lock up my tires?
« Reply #7 on: September 23, 2022, 07:50:44 AM »
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.
« Last Edit: September 23, 2022, 08:56:38 AM by MoparCarGuy »

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Re: Should I be able to lock up my tires?
« Reply #8 on: September 23, 2022, 03:28:32 PM »
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

Offline 70vert

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Re: Should I be able to lock up my tires?
« Reply #9 on: September 23, 2022, 04:50:47 PM »
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?

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Re: Should I be able to lock up my tires?
« Reply #10 on: September 23, 2022, 05:18:00 PM »
The LEED VP002 can achieve a little more than 24 in. Hg vacuum at sea level.

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Re: Should I be able to lock up my tires?
« Reply #10 on: September 23, 2022, 05:18:00 PM »

Offline Dakota

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Re: Should I be able to lock up my tires?
« Reply #11 on: September 26, 2022, 03:29:42 PM »
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 thatís 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.  Thereís 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!


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Re: Should I be able to lock up my tires?
« Reply #11 on: September 26, 2022, 03:29:42 PM »

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