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Old 10-14-2020, 11:08 PM   #1
BigTopGT
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Join Date: Apr 2019
Location: Fort Lauderdale, Florida 33311
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All new brake system: rears lock up and drag with weight in bed?

I'm not really sure where to look, at this point, but I'm beyond frustrated.

About 3 months ago I picked up a 71 LWB (350/350 auto) and on the test drive quickly realized the brakes went to the floor and there was almost no stopping power at all.

So, I YOLO'd it home 2 hours (all back roads or long, open highways across the middle of Florida) with the expectation of making it my daily driver. In the process of getting her to DD status, it took about a month to finally give up on the brakes and replace everything with new.

So, now it's got a Wilwood 4poston disc front, Wilwood 2.5" Drop spindles with a 1" CPP spring, Belltech shocks, Wilwood master cylinder, Wilwood proportioning valve, chrome double hump booster, stainless front lines, etc... And out back, I replaced the stock drums, wheel cylinders, soft lines, shoes, 5 inch springs, and matching shocks.

The only thing left from the original system is the hard lines.

So, with all that, the truck stops great, but ONLY if there's no weight in the bed.

The SECOND I throw even a few hundred pounds back there, and as soon as you hit the brakes, the rears lock up and eventually drag until they decide to release. (discovered this with a friend's 5.3 LS long block in the bed)

I'm not sure what to do after spending several thousand dollars on brakes, but the real concern here is it definitely can't tow even a small trailer, so I'm super frustrated.

If anyone has some tips as to where to look, I'm all ears and entirely appreciative.

Thanks!!! ��
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Old 10-14-2020, 11:37 PM   #2
geezer#99
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Re: All new brake system: rears lock up and drag with weight in bed?

Pics of your master and prop valve might help.

You might need an additional adjustable prop valve in the rear line.
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Old 10-15-2020, 08:08 AM   #3
ForcedInduction355
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Re: All new brake system: rears lock up and drag with weight in bed?

I agree with the above. I have heard of many disk/drum conversions where the rears lock up, and as mentioned the solution is usually an additional adjustable prop valve in the rear brake line. As far as them dragging, that sounds like some kind of residual pressure valve that is wrong for a drum application, but I'm not really sure there.

I am in the process of converting to 4 whl disk and hope it isn't an issue.

Wilwood should also be able to tell you what going on with their system and their prop valve.
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Old 10-15-2020, 09:43 AM   #4
jayoldschool
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Re: All new brake system: rears lock up and drag with weight in bed?

My bet is on "four wheel disc brake master cylinder" when it should be a disc/drum master.
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Old 10-15-2020, 10:17 AM   #5
72c20customcamper
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Re: All new brake system: rears lock up and drag with weight in bed?

Seems weird the rears would lock up when loaded only . If it was the portioning valve giving to much pressure they would lock all the time more so when empty. Same with the master. Could it be the parking brake is engaging when the rear drops a little? Could be causing the cable to tighten .
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Old 10-15-2020, 11:53 AM   #6
sick472
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Re: All new brake system: rears lock up and drag with weight in bed?

Quote:
Originally Posted by 72c20customcamper View Post
Seems weird the rears would lock up when loaded only . If it was the portioning valve giving to much pressure they would lock all the time more so when empty. Same with the master. Could it be the parking brake is engaging when the rear drops a little? Could be causing the cable to tighten .
^^^^^^^that's where I'd look first, That, and the rest of the brake system is a bit above my pay grade.
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Old 10-15-2020, 11:55 AM   #7
gmc684x4
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Re: All new brake system: rears lock up and drag with weight in bed?

Could the back brakes be dragging slightly at all times just more noticeable while carring extra weight

My question have you talked to wildwood about your setup ?
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Old 10-15-2020, 12:05 PM   #8
Richard
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Re: All new brake system: rears lock up and drag with weight in bed?

Saw you replaced the "soft lines" Any chance the rear is being kinked because of the drop?
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