We may have had some correspondence on this family before.
I'll try to make comments here----maybe others might be inclined to chime in.
From various sources, it appears that Samuel and Solomon were brothers and that Rachel was a sister.They appear, in turn, to be related to Robert and Elijah.
One problem is how to identify the various Robert Adamses there.The 1783 tax assessment for Bedford County shows three:Robert Adams senior, Robert Adams junior, and Robert Adams, younger.Robert Adams, junior, appears in a list in 1775, so he must have been legally an adult and married by that time.In order to reconcile these, I have tentatively made Robert Adams, Sr., be the father of Samuel (who died in 1781); Solomon; Rachel; and Robert (d. 1824).
Elijah (d. 1799) could have been the younger brother of the elder Robert.His son Robert was probably the Robert Adams, "younger," of 1783.This Robert moved away.Descendants (see posts on this board) say that Robert was born in 1753 and died in 1810 in Westmoreland County.He supposedly married Keziah Blair, who died 22 Mar. 1842, aged 84 years, in Porter Township, Jefferson County, PA.Robert was in Bedford County in 1790 but in 1800 was in Unity, Westmoreland Co.
Elijah's son Elijah married Hannah Blair, daughter of William and Hester Blair, and moved to Montgomery County, Kentucky by 1797 and died there soon after 1800.His children were raised by the mother's Blair relatives in Nicholas County, Kentucky (including my ancestor, Hester Adams).Elijah, Jr.'s children were born between 1785 and 1800.That might place his year of birth around 1760.
It is possible that Elijah, Sr., may have been born around 1730 and had two wives.The ones who received smaller amounts in his will could have been by the first wife (Robert, Elijah, Ruth), and the others could have been by Hannah.His son Ephraim Adams's will is genealogically significant.He died April 1819 in Unity Township, Westmoreland County, PA, leaving a wife Margaret and daughter Mariah.Among other bequests, he makes some to Ephraim Adams, son of Elijah and Hannah Adams of Kentucky; to Ephraim, son of Obadiah and Ruth Adams, of Bedford County; to Ephraim Smith, son of Joshuah and Margaret Smith; to Ephraim, son of James and Hannah Adams of Bedford County, and others.
The Dunning's Creek Monthly Meeting women's minutes say that Mary Smith, born 5 xi 1777, daughter of Robert and Dorothy Adams, married, on 20 xi 1820, Joseph Hewitt, of Napier Township.As some of the Adamses were Quakers, I think this Robert must be one of the family.However, for a long time I couldn't place him.However, I think this is Robert Adams, Sr.He may have married a woman named Dorothy as his second wife.
One other comment.Samuel Adams died before 5 Nov. 1781, when an inventory of his estate was made."Widow" Adams, probably his, was taxed in Quemahoning Township, Bedford County, in 1780-1781.William Proctor was appointed administrator of his estate.Storey's History of Cambria County, vol. 1, pp. 68-69, says that Samuel's widow married Jesse Proctor.As I can't find a Jesse Proctor, I think the name is a mistake---it probably should be William.They had a son, Isaac Proctor, born 1784, died November 1851 in Ravenswood, Jackson County, West Virginia.Samuel's son was Archibald (1764-1859).
It looks to me that Robert Adams might have been born around 1720 and died after 1800 in Bedford County.His younger brother Elijah was born perhaps about 1730.I agree on your suggested birth dates for Samuel and Solomon, but I think that Robert, Jr., could have been older.His son James married Mary Rose and had a son Edward, born in 1791 and was probably head of a household in 1790.This would make James born by 1765 or so, making Robert born perhaps around 1743.
I have looked at all the available Bedford County records.I've also looked in York and Adams County records, but without much success.It's possible that some Quaker records hold promise.The DNA results are promising.