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Patrick
Joined: 05 Dec 2007
Posts: 278
Location: Manila, PHILIPPINES
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 Posted: Thu Feb 13, 2014 12:36 pm Post subject: Rear left brake lockup in reverse,but not when going forward |
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Hello all!
When going in reverse, i step on the brake pedal with light to moderate pressure and the rear left side of my car goes up and down a good deal more than the rear right side, like maybe the rear left drumbrake locks sooner??
I`m thinking maybe the drum in question has its brake shoes wrongly adjusted?? But then, this is not the case when driving forward. Any ideas? I realize there is no brake proportioning valve, so...
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1979 924 U.S. version
Single sidedraft Weber 40
Last edited by Patrick on Thu Feb 13, 2014 8:50 pm; edited 1 time in total |
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jason c
Joined: 13 Jan 2014
Posts: 1018
Location: Nwi
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| Posted: Thu Feb 13, 2014 12:59 pm Post subject: |
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| One side is mid adjusted, the other isn't working or possible blown shock. |
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Slam
Joined: 07 Jan 2005
Posts: 1690
Location: Wainwright, Alberta, Canada
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| Posted: Thu Feb 13, 2014 3:06 pm Post subject: |
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Trailing pad on bad side misadjusted. It's too far out, so backing it off should do the trick. It's tricky balancing the two pads. Just my thoughts...
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'88 944 Auto - may or may not resurrect |
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jason c
Joined: 13 Jan 2014
Posts: 1018
Location: Nwi
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| Posted: Thu Feb 13, 2014 3:26 pm Post subject: |
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You'll need to pull the drums off & have a look. Free up the hardware, lube it and re-adjust the shoes.
Getting them adjusted properly can be tough, especially if the drums have a lip on them.
I had to fix the 4 wheel drum brakes on a car just like the one Kennedy was assassinated in, what a pita! |
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Patrick
Joined: 05 Dec 2007
Posts: 278
Location: Manila, PHILIPPINES
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| Posted: Thu Feb 13, 2014 8:45 pm Post subject: |
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Thanks for the replies guys!
I will check shoe adjustment on the "bad" side. I`m thinking though that a bad shock would cause a similiarly excessive rebound when braking from going forward as well. But it wont hurt to check the shock anyway.
By the way, anyone know what the official Porsche designation/description is of the drum brakes? Haynes calls it a "Simplex (Single leading shoe)" I`d like to understand more how it works. Cheers!
patrick
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1979 924 U.S. version
Single sidedraft Weber 40 |
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Rasta Monsta
Joined: 12 Jul 2006
Posts: 11733
Location: PacNW
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| Posted: Fri Feb 14, 2014 2:15 am Post subject: |
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I suggest pulling both drums, taking everything apart, cleaning carefully, lubricating all contact points in the mechanism (including spots where the shoes slide on the backing plates) and reassembling.
If you get it looking like new, you have a far better chance of it working like new.
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Toofah King Bad
- WeiBe (1987 924S 2.5t) - 931 S3
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!tom
Joined: 28 Aug 2006
Posts: 1941
Location: Victoria, BC Canada
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| Posted: Fri Feb 14, 2014 4:40 pm Post subject: |
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| Patrick wrote: | | By the way, anyone know what the official Porsche designation/description is of the drum brakes? Haynes calls it a "Simplex (Single leading shoe)" I`d like to understand more how it works. |
Most drum brakes have the wheel cylinder at the top which expands two shoes. The shoes at the bottom are normally connected to one another, but do not connect to the backing plate. So, the trailing shoe tends to be the anchor at the wheel cylinder when the brakes are energized.
Given this arrangement, both shoes tend to self energize, due to the fact the anchor points of each shoe are at a smaller radius than the friction surface. During normal service while moving forward, the top of the trailing shoe (next to the wheel cylinder) will have minimal movement, whereas the other side of the wheel cylinder will take up all of the clearance.
The 924 has a different arrangement in that both shoes anchor to the backing plate at the bottom. The leading shoe will behave similarly to the leading shoe in a conventional arrangement, but the trailing shoe will behave quite differently due to the fact that it will not be anchoring the leading shoe. Both shoes will have similar movement with respect to the wheel cylinder at the top due to the fact both are anchored at the bottom.
Given this arrangement, my understanding is that only the leading shoe would tend to self energize, which will tend to give better brake modulation characteristics than a conventional drum brake arrangement.
Motorcycles with front drum brakes (yes, very old) tended to have an arrangement where both shoes behaved like the trailing shoe in the 924 arrangement in order to improve brake modulation characteristics at the expense of ultimate brake torque.
So, the 924 drum brakes have 4 adjusters, one for each shoe. Conventional drum brakes have one per pair of shoes.
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78 924 NA
5-lug |
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