Many secondary sources give John a middle initial "G," but so far, I've found no primary record where he was shown with a middle initial.

1.  Marriage Record:

2.  1830 Census Index and Digital Images (online at Ancestry.com; Image #8 of 9; extracted by Diana Gale Matthiesen):¤•

1830

PA

Huntingdon Co.

Walker [now Juniata] Twp.

Row 166

p. 113

Ln 4

John Corban

200 201 - 010 010

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
2 males	4 or under	1825-1830	= David (b. 1827/8) = William (b. 1826/7)
2 males	15-19	1810-1815	= ? = ?
1 male	30-39	1790-1800	= John (b. 1794/5)
1 female	5-9	1820-1825	= Jane (b. 1824/5)
1 female	20-29	1800-1810	= Elizabeth (b. 1804)

The two teenage males are too old to be our subjects' children.  Listed two lines from John's brother, David G. CORBIN. 

3.  1840 Census Index and Digital Images (online at Ancestry.com; Image #9 of 12; extracted by Diana Gale Matthiesen):¤•

1840

PA

Huntingdon Co.

Walker [now Juniata] Twp.

Roll 462

pp. 394B-395A

John Corbin

302 000 001 - 120 100 010

0100000

These data indicate:

No. & Sex	Age Class	Therefore Born	Individuals Inferred
3 males	4 or under	1835-1840	= Son C = Son B = Son A
2 males	10-14	1825-1830	= David (b. 1827/8) = William (b. 1826/7)
1 male	60-69	1770-1780	= John (b. 1794/5)
1 female	4 or under	1835-1840	= Mary (b. 1835/6)
2 females	5-9	1830-1835	= Sarah (b. 1833/4) = Daughter A
1 female	15-19	1820-1825	= Jane (b. 1824/5)
1 female	50-59	1780-1790	= Elizabeth (b. 1804)
1	person engaged in agriculture

John and Elizabeth are in the wrong age-classes, but there can be no doubt these are our subjects.  Listed two lines from John's brother, David G. CORBIN. 

4.  1850 Census Index/Microfilm (online at GenealogyLibrary.com):  Walker [now Juniata] Twp., Huntingdon Co., PA, p. 168, 153/157, enumerated 23 Oct 1850 (extracted by Diana Gale Matthiesen):¤•

John Corban	55	M	Farmer	2000	Penna
Elizabeth "	40	F			"
Jane      "	25	F			"
William   "	23	M	Farmer		"
David     "	22	M			"
Sarah     "	16	F			"
Mary      "	14	F			"
Calvin    "	8	M			"
Ellen     "	4	F			"

Listed two households from John's nephew, Abraham CORBIN.

5.  1860 Census Images (online at Ancestry.com; Image #9 of 12):  Huntingdon P.O., Juniata Twp., Huntingdon Co., PA, Roll 1115 (Book 1), p. 353, PN 9, 66/61, enumerated 15 Jun 1860, official enumeration date 1 Jun 1860 (extracted by Diana Gale Matthiesen):¤•

John Corbin	63	M		Farmer	200	300	Penna
Elizabeth "	56	F					Do
Calvin    "	18	M		Laborer			Do

Listed next to their daughter and son-in-law, George & Mary (CORBIN) MOUNTAIN. 

6.  1870 Census Images (online at Ancestry.com; Image #6 of 10):  McConnellstown P.O., Juniata Twp., Huntingdon Co., PA, p. 647B, PN 6, 39/40, enumerated 29 Jun 1870, official enumeration date 1 Jun 1870 (extracted by Diana Gale Matthiesen):¤•

Mountain Geo R	38	M	W	Farmer	1600	878	Pa
________ Mary A	32	F	W	Keeping house			Pa
[George & Mary's children]
Corbin Elizabeth	65	F	W				Pa
Ridenouer Jane	16	F	W	Dom Ser			Pa

Jane is kin, but I don't know how she connects (Elizabeth's sister-in-law married a REIDENOUR). 

7.  1880 United States Census and National Index (LDS Family History Resource File: CD-ROM Library):¤•

Census Place: Juniata, Huntingdon, Pennsylvania
Source: FHL Film 1255134; NARA Film T9-1134; Pg 203D
	Relation	Sex	Marr	Race	Age	Birthplace	Occupation	Fa	Mo
George R. MOUNTAIN	Self	M	M	W	46	PA	Farmer	PA	PA
Mary A. MOUNTAIN	Wife	F	M	W	44	PA	Keeping House	PA	PA
[George & Mary's children]
Elizabeth CORBIN	MotherL	F	W	W	75	PA	Keeping House	PA	PA

8.  WorldConnect / Ancestry World Trees (online at RootsWeb.com/Ancestry.com).